[PATCH v7 1/2] mm: introduce process_mrelease system call

[PATCH v7 1/2] mm: introduce process_mrelease system call

[Suren Baghdasaryan]

In modern systems it's not unusual to have a system component monitoring
memory conditions of the system and tasked with keeping system memory
pressure under control. One way to accomplish that is to kill
non-essential processes to free up memory for more important ones.
Examples of this are Facebook's OOM killer daemon called oomd and
Android's low memory killer daemon called lmkd.
For such system component it's important to be able to free memory
quickly and efficiently. Unfortunately the time process takes to free
up its memory after receiving a SIGKILL might vary based on the state
of the process (uninterruptible sleep), size and OPP level of the core
the process is running. A mechanism to free resources of the target
process in a more predictable way would improve system's ability to
control its memory pressure.
Introduce process_mrelease system call that releases memory of a dying
process from the context of the caller. This way the memory is freed in
a more controllable way with CPU affinity and priority of the caller.
The workload of freeing the memory will also be charged to the caller.
The operation is allowed only on a dying process.

After previous discussions [1, 2, 3] the decision was made [4] to introduce
a dedicated system call to cover this use case.

The API is as follows,

          int process_mrelease(int pidfd, unsigned int flags);

        DESCRIPTION
          The process_mrelease() system call is used to free the memory of
          an exiting process.

          The pidfd selects the process referred to by the PID file
          descriptor.
          (See pidfd_open(2) for further information)

          The flags argument is reserved for future use; currently, this
          argument must be specified as 0.

        RETURN VALUE
          On success, process_mrelease() returns 0. On error, -1 is
          returned and errno is set to indicate the error.

        ERRORS
          EBADF  pidfd is not a valid PID file descriptor.

          EAGAIN Failed to release part of the address space.

          EINTR  The call was interrupted by a signal; see signal(7).

          EINVAL flags is not 0.

          EINVAL The memory of the task cannot be released because the
                 process is not exiting, the address space is shared
                 with another live process or there is a core dump in
                 progress.

          ENOSYS This system call is not supported, for example, without
                 MMU support built into Linux.

          ESRCH  The target process does not exist (i.e., it has terminated
                 and been waited on).

[1] https://lore.kernel.org/lkml/20190411014353.113252-3-surenb@google.com/
[2] https://lore.kernel.org/linux-api/20201113173448.1863419-1-surenb@google.com/
[3] https://lore.kernel.org/linux-api/20201124053943.1684874-3-surenb@google.com/
[4] https://lore.kernel.org/linux-api/20201223075712.GA4719@lst.de/

Signed-off-by: Suren Baghdasaryan <surenb@google.com>
---
changes in v7:
- Fixed pidfd_open misspelling, per Andrew Morton
- Fixed wrong task pinning after find_lock_task_mm() issue, per Michal Hocko
- Moved MMF_OOM_SKIP check before task_will_free_mem(), per Michal Hocko

 mm/oom_kill.c | 73 +++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 73 insertions(+)

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index c729a4c4a1ac..a4d917b43c73 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -28,6 +28,7 @@
 #include <linux/sched/task.h>
 #include <linux/sched/debug.h>
 #include <linux/swap.h>
+#include <linux/syscalls.h>
 #include <linux/timex.h>
 #include <linux/jiffies.h>
 #include <linux/cpuset.h>
@@ -1141,3 +1142,75 @@ void pagefault_out_of_memory(void)
 	out_of_memory(&oc);
 	mutex_unlock(&oom_lock);
 }
+
+SYSCALL_DEFINE2(process_mrelease, int, pidfd, unsigned int, flags)
+{
+#ifdef CONFIG_MMU
+	struct mm_struct *mm = NULL;
+	struct task_struct *task;
+	struct task_struct *p;
+	unsigned int f_flags;
+	struct pid *pid;
+	long ret = 0;
+
+	if (flags)
+		return -EINVAL;
+
+	pid = pidfd_get_pid(pidfd, &f_flags);
+	if (IS_ERR(pid))
+		return PTR_ERR(pid);
+
+	task = get_pid_task(pid, PIDTYPE_PID);
+	if (!task) {
+		ret = -ESRCH;
+		goto put_pid;
+	}
+
+	/*
+	 * If the task is dying and in the process of releasing its memory
+	 * then get its mm.
+	 */
+	p = find_lock_task_mm(task);
+	if (!p) {
+		ret = -ESRCH;
+		goto put_pid;
+	}
+	if (task != p) {
+		get_task_struct(p);
+		put_task_struct(task);
+		task = p;
+	}
+
+	/* If the work has been done already, just exit with success */
+	if (test_bit(MMF_OOM_SKIP, &task->mm->flags))
+		goto put_task;
+
+	if (task_will_free_mem(task) && (task->flags & PF_KTHREAD) == 0) {
+		mm = task->mm;
+		mmget(mm);
+	}
+	task_unlock(task);
+	if (!mm) {
+		ret = -EINVAL;
+		goto put_task;
+	}
+
+	if (mmap_read_lock_killable(mm)) {
+		ret = -EINTR;
+		goto put_mm;
+	}
+	if (!__oom_reap_task_mm(mm))
+		ret = -EAGAIN;
+	mmap_read_unlock(mm);
+
+put_mm:
+	mmput(mm);
+put_task:
+	put_task_struct(task);
+put_pid:
+	put_pid(pid);
+	return ret;
+#else
+	return -ENOSYS;
+#endif /* CONFIG_MMU */
+}
-- 
2.32.0.554.ge1b32706d8-goog

[PATCH v7 2/2] mm: wire up syscall process_mrelease

[Suren Baghdasaryan]

Split off from prev patch in the series that implements the syscall.

Signed-off-by: Suren Baghdasaryan <surenb@google.com>
---
 arch/alpha/kernel/syscalls/syscall.tbl      | 2 ++
 arch/arm/tools/syscall.tbl                  | 2 ++
 arch/arm64/include/asm/unistd.h             | 2 +-
 arch/arm64/include/asm/unistd32.h           | 2 ++
 arch/ia64/kernel/syscalls/syscall.tbl       | 2 ++
 arch/m68k/kernel/syscalls/syscall.tbl       | 2 ++
 arch/microblaze/kernel/syscalls/syscall.tbl | 2 ++
 arch/mips/kernel/syscalls/syscall_n32.tbl   | 2 ++
 arch/mips/kernel/syscalls/syscall_n64.tbl   | 2 ++
 arch/mips/kernel/syscalls/syscall_o32.tbl   | 2 ++
 arch/parisc/kernel/syscalls/syscall.tbl     | 2 ++
 arch/powerpc/kernel/syscalls/syscall.tbl    | 2 ++
 arch/s390/kernel/syscalls/syscall.tbl       | 2 ++
 arch/sh/kernel/syscalls/syscall.tbl         | 2 ++
 arch/sparc/kernel/syscalls/syscall.tbl      | 2 ++
 arch/x86/entry/syscalls/syscall_32.tbl      | 1 +
 arch/x86/entry/syscalls/syscall_64.tbl      | 1 +
 arch/xtensa/kernel/syscalls/syscall.tbl     | 2 ++
 include/linux/syscalls.h                    | 1 +
 include/uapi/asm-generic/unistd.h           | 4 +++-
 kernel/sys_ni.c                             | 1 +
 21 files changed, 38 insertions(+), 2 deletions(-)

diff --git a/arch/alpha/kernel/syscalls/syscall.tbl b/arch/alpha/kernel/syscalls/syscall.tbl
index a17687ed4b51..605645eae04c 100644
--- a/arch/alpha/kernel/syscalls/syscall.tbl
+++ b/arch/alpha/kernel/syscalls/syscall.tbl
@@ -486,3 +486,5 @@
 554	common	landlock_create_ruleset		sys_landlock_create_ruleset
 555	common	landlock_add_rule		sys_landlock_add_rule
 556	common	landlock_restrict_self		sys_landlock_restrict_self
+# 557 reserved for memfd_secret
+558	common	process_mrelease		sys_process_mrelease
diff --git a/arch/arm/tools/syscall.tbl b/arch/arm/tools/syscall.tbl
index c5df1179fc5d..2f32eb8beca8 100644
--- a/arch/arm/tools/syscall.tbl
+++ b/arch/arm/tools/syscall.tbl
@@ -460,3 +460,5 @@
 444	common	landlock_create_ruleset		sys_landlock_create_ruleset
 445	common	landlock_add_rule		sys_landlock_add_rule
 446	common	landlock_restrict_self		sys_landlock_restrict_self
+# 447 reserved for memfd_secret
+448	common	process_mrelease		sys_process_mrelease
diff --git a/arch/arm64/include/asm/unistd.h b/arch/arm64/include/asm/unistd.h
index 727bfc3be99b..3cb206aea3db 100644
--- a/arch/arm64/include/asm/unistd.h
+++ b/arch/arm64/include/asm/unistd.h
@@ -38,7 +38,7 @@
 #define __ARM_NR_compat_set_tls		(__ARM_NR_COMPAT_BASE + 5)
 #define __ARM_NR_COMPAT_END		(__ARM_NR_COMPAT_BASE + 0x800)
 
-#define __NR_compat_syscalls		447
+#define __NR_compat_syscalls		449
 #endif
 
 #define __ARCH_WANT_SYS_CLONE
diff --git a/arch/arm64/include/asm/unistd32.h b/arch/arm64/include/asm/unistd32.h
index 99ffcafc736c..0f49cdb180dd 100644
--- a/arch/arm64/include/asm/unistd32.h
+++ b/arch/arm64/include/asm/unistd32.h
@@ -901,6 +901,8 @@ __SYSCALL(__NR_landlock_create_ruleset, sys_landlock_create_ruleset)
 __SYSCALL(__NR_landlock_add_rule, sys_landlock_add_rule)
 #define __NR_landlock_restrict_self 446
 __SYSCALL(__NR_landlock_restrict_self, sys_landlock_restrict_self)
+#define __NR_process_mrelease 448
+__SYSCALL(__NR_process_mrelease, sys_process_mrelease)
 
 /*
  * Please add new compat syscalls above this comment and update
diff --git a/arch/ia64/kernel/syscalls/syscall.tbl b/arch/ia64/kernel/syscalls/syscall.tbl
index 6d07742c57b8..9bf45f2be966 100644
--- a/arch/ia64/kernel/syscalls/syscall.tbl
+++ b/arch/ia64/kernel/syscalls/syscall.tbl
@@ -367,3 +367,5 @@
 444	common	landlock_create_ruleset		sys_landlock_create_ruleset
 445	common	landlock_add_rule		sys_landlock_add_rule
 446	common	landlock_restrict_self		sys_landlock_restrict_self
+# 447 reserved for memfd_secret
+448	common	process_mrelease		sys_process_mrelease
diff --git a/arch/m68k/kernel/syscalls/syscall.tbl b/arch/m68k/kernel/syscalls/syscall.tbl
index 541bc1b3a8f9..f1f98ee6c82d 100644
--- a/arch/m68k/kernel/syscalls/syscall.tbl
+++ b/arch/m68k/kernel/syscalls/syscall.tbl
@@ -446,3 +446,5 @@
 444	common	landlock_create_ruleset		sys_landlock_create_ruleset
 445	common	landlock_add_rule		sys_landlock_add_rule
 446	common	landlock_restrict_self		sys_landlock_restrict_self
+# 447 reserved for memfd_secret
+448	common	process_mrelease		sys_process_mrelease
diff --git a/arch/microblaze/kernel/syscalls/syscall.tbl b/arch/microblaze/kernel/syscalls/syscall.tbl
index a176faca2927..da49ddd4bb54 100644
--- a/arch/microblaze/kernel/syscalls/syscall.tbl
+++ b/arch/microblaze/kernel/syscalls/syscall.tbl
@@ -452,3 +452,5 @@
 444	common	landlock_create_ruleset		sys_landlock_create_ruleset
 445	common	landlock_add_rule		sys_landlock_add_rule
 446	common	landlock_restrict_self		sys_landlock_restrict_self
+# 447 reserved for memfd_secret
+448	common	process_mrelease		sys_process_mrelease
diff --git a/arch/mips/kernel/syscalls/syscall_n32.tbl b/arch/mips/kernel/syscalls/syscall_n32.tbl
index c2d2e19abea8..56c8d3cf42ed 100644
--- a/arch/mips/kernel/syscalls/syscall_n32.tbl
+++ b/arch/mips/kernel/syscalls/syscall_n32.tbl
@@ -385,3 +385,5 @@
 444	n32	landlock_create_ruleset		sys_landlock_create_ruleset
 445	n32	landlock_add_rule		sys_landlock_add_rule
 446	n32	landlock_restrict_self		sys_landlock_restrict_self
+# 447 reserved for memfd_secret
+448	n32	process_mrelease		sys_process_mrelease
diff --git a/arch/mips/kernel/syscalls/syscall_n64.tbl b/arch/mips/kernel/syscalls/syscall_n64.tbl
index ac653d08b1ea..1ca7bc337932 100644
--- a/arch/mips/kernel/syscalls/syscall_n64.tbl
+++ b/arch/mips/kernel/syscalls/syscall_n64.tbl
@@ -361,3 +361,5 @@
 444	n64	landlock_create_ruleset		sys_landlock_create_ruleset
 445	n64	landlock_add_rule		sys_landlock_add_rule
 446	n64	landlock_restrict_self		sys_landlock_restrict_self
+# 447 reserved for memfd_secret
+448	n64	process_mrelease		sys_process_mrelease
diff --git a/arch/mips/kernel/syscalls/syscall_o32.tbl b/arch/mips/kernel/syscalls/syscall_o32.tbl
index 253f2cd70b6b..fd3a9df60ec2 100644
--- a/arch/mips/kernel/syscalls/syscall_o32.tbl
+++ b/arch/mips/kernel/syscalls/syscall_o32.tbl
@@ -434,3 +434,5 @@
 444	o32	landlock_create_ruleset		sys_landlock_create_ruleset
 445	o32	landlock_add_rule		sys_landlock_add_rule
 446	o32	landlock_restrict_self		sys_landlock_restrict_self
+# 447 reserved for memfd_secret
+448	o32	process_mrelease		sys_process_mrelease
diff --git a/arch/parisc/kernel/syscalls/syscall.tbl b/arch/parisc/kernel/syscalls/syscall.tbl
index e26187b9ab87..040df1b7a589 100644
--- a/arch/parisc/kernel/syscalls/syscall.tbl
+++ b/arch/parisc/kernel/syscalls/syscall.tbl
@@ -444,3 +444,5 @@
 444	common	landlock_create_ruleset		sys_landlock_create_ruleset
 445	common	landlock_add_rule		sys_landlock_add_rule
 446	common	landlock_restrict_self		sys_landlock_restrict_self
+# 447 reserved for memfd_secret
+448	common	process_mrelease		sys_process_mrelease
diff --git a/arch/powerpc/kernel/syscalls/syscall.tbl b/arch/powerpc/kernel/syscalls/syscall.tbl
index aef2a290e71a..d8ebd7d37c0f 100644
--- a/arch/powerpc/kernel/syscalls/syscall.tbl
+++ b/arch/powerpc/kernel/syscalls/syscall.tbl
@@ -526,3 +526,5 @@
 444	common	landlock_create_ruleset		sys_landlock_create_ruleset
 445	common	landlock_add_rule		sys_landlock_add_rule
 446	common	landlock_restrict_self		sys_landlock_restrict_self
+# 447 reserved for memfd_secret
+448	common	process_mrelease		sys_process_mrelease
diff --git a/arch/s390/kernel/syscalls/syscall.tbl b/arch/s390/kernel/syscalls/syscall.tbl
index 64d51ab5a8b4..57233ace30cb 100644
--- a/arch/s390/kernel/syscalls/syscall.tbl
+++ b/arch/s390/kernel/syscalls/syscall.tbl
@@ -449,3 +449,5 @@
 444  common	landlock_create_ruleset	sys_landlock_create_ruleset	sys_landlock_create_ruleset
 445  common	landlock_add_rule	sys_landlock_add_rule		sys_landlock_add_rule
 446  common	landlock_restrict_self	sys_landlock_restrict_self	sys_landlock_restrict_self
+# 447 reserved for memfd_secret
+448  common	process_mrelease	sys_process_mrelease		sys_process_mrelease
diff --git a/arch/sh/kernel/syscalls/syscall.tbl b/arch/sh/kernel/syscalls/syscall.tbl
index e0a70be77d84..2f6e95eb4690 100644
--- a/arch/sh/kernel/syscalls/syscall.tbl
+++ b/arch/sh/kernel/syscalls/syscall.tbl
@@ -449,3 +449,5 @@
 444	common	landlock_create_ruleset		sys_landlock_create_ruleset
 445	common	landlock_add_rule		sys_landlock_add_rule
 446	common	landlock_restrict_self		sys_landlock_restrict_self
+# 447 reserved for memfd_secret
+448	common	process_mrelease		sys_process_mrelease
diff --git a/arch/sparc/kernel/syscalls/syscall.tbl b/arch/sparc/kernel/syscalls/syscall.tbl
index 603f5a821502..42fc2906215d 100644
--- a/arch/sparc/kernel/syscalls/syscall.tbl
+++ b/arch/sparc/kernel/syscalls/syscall.tbl
@@ -492,3 +492,5 @@
 444	common	landlock_create_ruleset		sys_landlock_create_ruleset
 445	common	landlock_add_rule		sys_landlock_add_rule
 446	common	landlock_restrict_self		sys_landlock_restrict_self
+# 447 reserved for memfd_secret
+448	common	process_mrelease		sys_process_mrelease
diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
index ce763a12311c..661a03bcfbd1 100644
--- a/arch/x86/entry/syscalls/syscall_32.tbl
+++ b/arch/x86/entry/syscalls/syscall_32.tbl
@@ -452,3 +452,4 @@
 445	i386	landlock_add_rule	sys_landlock_add_rule
 446	i386	landlock_restrict_self	sys_landlock_restrict_self
 447	i386	memfd_secret		sys_memfd_secret
+448	i386	process_mrelease	sys_process_mrelease
diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
index f6b57799c1ea..807b6a1de8e8 100644
--- a/arch/x86/entry/syscalls/syscall_64.tbl
+++ b/arch/x86/entry/syscalls/syscall_64.tbl
@@ -369,6 +369,7 @@
 445	common	landlock_add_rule	sys_landlock_add_rule
 446	common	landlock_restrict_self	sys_landlock_restrict_self
 447	common	memfd_secret		sys_memfd_secret
+448	common	process_mrelease	sys_process_mrelease
 
 #
 # Due to a historical design error, certain syscalls are numbered differently
diff --git a/arch/xtensa/kernel/syscalls/syscall.tbl b/arch/xtensa/kernel/syscalls/syscall.tbl
index 235d67d6ceb4..f4384951f393 100644
--- a/arch/xtensa/kernel/syscalls/syscall.tbl
+++ b/arch/xtensa/kernel/syscalls/syscall.tbl
@@ -417,3 +417,5 @@
 444	common	landlock_create_ruleset		sys_landlock_create_ruleset
 445	common	landlock_add_rule		sys_landlock_add_rule
 446	common	landlock_restrict_self		sys_landlock_restrict_self
+# 447 reserved for memfd_secret
+448	common	process_mrelease		sys_process_mrelease
diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index 69c9a7010081..00bc170a50f0 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -915,6 +915,7 @@ asmlinkage long sys_mincore(unsigned long start, size_t len,
 asmlinkage long sys_madvise(unsigned long start, size_t len, int behavior);
 asmlinkage long sys_process_madvise(int pidfd, const struct iovec __user *vec,
 			size_t vlen, int behavior, unsigned int flags);
+asmlinkage long sys_process_mrelease(int pidfd, unsigned int flags);
 asmlinkage long sys_remap_file_pages(unsigned long start, unsigned long size,
 			unsigned long prot, unsigned long pgoff,
 			unsigned long flags);
diff --git a/include/uapi/asm-generic/unistd.h b/include/uapi/asm-generic/unistd.h
index a9d6fcd95f42..14c8fe863c6d 100644
--- a/include/uapi/asm-generic/unistd.h
+++ b/include/uapi/asm-generic/unistd.h
@@ -877,9 +877,11 @@ __SYSCALL(__NR_landlock_restrict_self, sys_landlock_restrict_self)
 #define __NR_memfd_secret 447
 __SYSCALL(__NR_memfd_secret, sys_memfd_secret)
 #endif
+#define __NR_process_mrelease 448
+__SYSCALL(__NR_process_mrelease, sys_process_mrelease)
 
 #undef __NR_syscalls
-#define __NR_syscalls 448
+#define __NR_syscalls 449
 
 /*
  * 32 bit systems traditionally used different
diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
index 30971b1dd4a9..18a9c2cde767 100644
--- a/kernel/sys_ni.c
+++ b/kernel/sys_ni.c
@@ -289,6 +289,7 @@ COND_SYSCALL(munlockall);
 COND_SYSCALL(mincore);
 COND_SYSCALL(madvise);
 COND_SYSCALL(process_madvise);
+COND_SYSCALL(process_mrelease);
 COND_SYSCALL(remap_file_pages);
 COND_SYSCALL(mbind);
 COND_SYSCALL_COMPAT(mbind);
-- 
2.32.0.554.ge1b32706d8-goog

Re: [PATCH v7 1/2] mm: introduce process_mrelease system call

[David Hildenbrand]

On 05.08.21 19:08, Suren Baghdasaryan wrote:
> In modern systems it's not unusual to have a system component monitoring
> memory conditions of the system and tasked with keeping system memory
> pressure under control. One way to accomplish that is to kill
> non-essential processes to free up memory for more important ones.
> Examples of this are Facebook's OOM killer daemon called oomd and
> Android's low memory killer daemon called lmkd.
> For such system component it's important to be able to free memory
> quickly and efficiently. Unfortunately the time process takes to free
> up its memory after receiving a SIGKILL might vary based on the state
> of the process (uninterruptible sleep), size and OPP level of the core
> the process is running. A mechanism to free resources of the target
> process in a more predictable way would improve system's ability to
> control its memory pressure.
> Introduce process_mrelease system call that releases memory of a dying
> process from the context of the caller. This way the memory is freed in
> a more controllable way with CPU affinity and priority of the caller.
> The workload of freeing the memory will also be charged to the caller.
> The operation is allowed only on a dying process.
> 
> After previous discussions [1, 2, 3] the decision was made [4] to introduce
> a dedicated system call to cover this use case.
> 
> The API is as follows,
> 
>            int process_mrelease(int pidfd, unsigned int flags);
> 
>          DESCRIPTION
>            The process_mrelease() system call is used to free the memory of
>            an exiting process.
> 
>            The pidfd selects the process referred to by the PID file
>            descriptor.
>            (See pidfd_open(2) for further information)
> 
>            The flags argument is reserved for future use; currently, this
>            argument must be specified as 0.
> 
>          RETURN VALUE
>            On success, process_mrelease() returns 0. On error, -1 is
>            returned and errno is set to indicate the error.
> 
>          ERRORS
>            EBADF  pidfd is not a valid PID file descriptor.
> 
>            EAGAIN Failed to release part of the address space.
> 
>            EINTR  The call was interrupted by a signal; see signal(7).
> 
>            EINVAL flags is not 0.
> 
>            EINVAL The memory of the task cannot be released because the
>                   process is not exiting, the address space is shared
>                   with another live process or there is a core dump in
>                   progress.
> 
>            ENOSYS This system call is not supported, for example, without
>                   MMU support built into Linux.
> 
>            ESRCH  The target process does not exist (i.e., it has terminated
>                   and been waited on).
> 
> [1] https://lore.kernel.org/lkml/20190411014353.113252-3-surenb@google.com/
> [2] https://lore.kernel.org/linux-api/20201113173448.1863419-1-surenb@google.com/
> [3] https://lore.kernel.org/linux-api/20201124053943.1684874-3-surenb@google.com/
> [4] https://lore.kernel.org/linux-api/20201223075712.GA4719@lst.de/
> 
> Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> ---
> changes in v7:
> - Fixed pidfd_open misspelling, per Andrew Morton
> - Fixed wrong task pinning after find_lock_task_mm() issue, per Michal Hocko
> - Moved MMF_OOM_SKIP check before task_will_free_mem(), per Michal Hocko
> 
>   mm/oom_kill.c | 73 +++++++++++++++++++++++++++++++++++++++++++++++++++
>   1 file changed, 73 insertions(+)
> 
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index c729a4c4a1ac..a4d917b43c73 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -28,6 +28,7 @@
>   #include <linux/sched/task.h>
>   #include <linux/sched/debug.h>
>   #include <linux/swap.h>
> +#include <linux/syscalls.h>
>   #include <linux/timex.h>
>   #include <linux/jiffies.h>
>   #include <linux/cpuset.h>
> @@ -1141,3 +1142,75 @@ void pagefault_out_of_memory(void)
>   	out_of_memory(&oc);
>   	mutex_unlock(&oom_lock);
>   }
> +
> +SYSCALL_DEFINE2(process_mrelease, int, pidfd, unsigned int, flags)
> +{
> +#ifdef CONFIG_MMU
> +	struct mm_struct *mm = NULL;
> +	struct task_struct *task;
> +	struct task_struct *p;
> +	unsigned int f_flags;
> +	struct pid *pid;
> +	long ret = 0;
> +
> +	if (flags)
> +		return -EINVAL;
> +
> +	pid = pidfd_get_pid(pidfd, &f_flags);
> +	if (IS_ERR(pid))
> +		return PTR_ERR(pid);
> +
> +	task = get_pid_task(pid, PIDTYPE_PID);
> +	if (!task) {
> +		ret = -ESRCH;
> +		goto put_pid;
> +	}
> +
> +	/*
> +	 * If the task is dying and in the process of releasing its memory
> +	 * then get its mm.
> +	 */
> +	p = find_lock_task_mm(task);
> +	if (!p) {
> +		ret = -ESRCH;
> +		goto put_pid;
> +	}
> +	if (task != p) {
> +		get_task_struct(p);


Wouldn't we want to obtain the mm from p ? I thought that was the whole 
exercise of going via find_lock_task_mm().

-- 
Thanks,

David / dhildenb

Re: [PATCH v7 1/2] mm: introduce process_mrelease system call

[Suren Baghdasaryan]

On Thu, Aug 5, 2021 at 10:29 AM David Hildenbrand <david@redhat.com> wrote:
>
> On 05.08.21 19:08, Suren Baghdasaryan wrote:
> > In modern systems it's not unusual to have a system component monitoring
> > memory conditions of the system and tasked with keeping system memory
> > pressure under control. One way to accomplish that is to kill
> > non-essential processes to free up memory for more important ones.
> > Examples of this are Facebook's OOM killer daemon called oomd and
> > Android's low memory killer daemon called lmkd.
> > For such system component it's important to be able to free memory
> > quickly and efficiently. Unfortunately the time process takes to free
> > up its memory after receiving a SIGKILL might vary based on the state
> > of the process (uninterruptible sleep), size and OPP level of the core
> > the process is running. A mechanism to free resources of the target
> > process in a more predictable way would improve system's ability to
> > control its memory pressure.
> > Introduce process_mrelease system call that releases memory of a dying
> > process from the context of the caller. This way the memory is freed in
> > a more controllable way with CPU affinity and priority of the caller.
> > The workload of freeing the memory will also be charged to the caller.
> > The operation is allowed only on a dying process.
> >
> > After previous discussions [1, 2, 3] the decision was made [4] to introduce
> > a dedicated system call to cover this use case.
> >
> > The API is as follows,
> >
> >            int process_mrelease(int pidfd, unsigned int flags);
> >
> >          DESCRIPTION
> >            The process_mrelease() system call is used to free the memory of
> >            an exiting process.
> >
> >            The pidfd selects the process referred to by the PID file
> >            descriptor.
> >            (See pidfd_open(2) for further information)
> >
> >            The flags argument is reserved for future use; currently, this
> >            argument must be specified as 0.
> >
> >          RETURN VALUE
> >            On success, process_mrelease() returns 0. On error, -1 is
> >            returned and errno is set to indicate the error.
> >
> >          ERRORS
> >            EBADF  pidfd is not a valid PID file descriptor.
> >
> >            EAGAIN Failed to release part of the address space.
> >
> >            EINTR  The call was interrupted by a signal; see signal(7).
> >
> >            EINVAL flags is not 0.
> >
> >            EINVAL The memory of the task cannot be released because the
> >                   process is not exiting, the address space is shared
> >                   with another live process or there is a core dump in
> >                   progress.
> >
> >            ENOSYS This system call is not supported, for example, without
> >                   MMU support built into Linux.
> >
> >            ESRCH  The target process does not exist (i.e., it has terminated
> >                   and been waited on).
> >
> > [1] https://lore.kernel.org/lkml/20190411014353.113252-3-surenb@google.com/
> > [2] https://lore.kernel.org/linux-api/20201113173448.1863419-1-surenb@google.com/
> > [3] https://lore.kernel.org/linux-api/20201124053943.1684874-3-surenb@google.com/
> > [4] https://lore.kernel.org/linux-api/20201223075712.GA4719@lst.de/
> >
> > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > ---
> > changes in v7:
> > - Fixed pidfd_open misspelling, per Andrew Morton
> > - Fixed wrong task pinning after find_lock_task_mm() issue, per Michal Hocko
> > - Moved MMF_OOM_SKIP check before task_will_free_mem(), per Michal Hocko
> >
> >   mm/oom_kill.c | 73 +++++++++++++++++++++++++++++++++++++++++++++++++++
> >   1 file changed, 73 insertions(+)
> >
> > diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> > index c729a4c4a1ac..a4d917b43c73 100644
> > --- a/mm/oom_kill.c
> > +++ b/mm/oom_kill.c
> > @@ -28,6 +28,7 @@
> >   #include <linux/sched/task.h>
> >   #include <linux/sched/debug.h>
> >   #include <linux/swap.h>
> > +#include <linux/syscalls.h>
> >   #include <linux/timex.h>
> >   #include <linux/jiffies.h>
> >   #include <linux/cpuset.h>
> > @@ -1141,3 +1142,75 @@ void pagefault_out_of_memory(void)
> >       out_of_memory(&oc);
> >       mutex_unlock(&oom_lock);
> >   }
> > +
> > +SYSCALL_DEFINE2(process_mrelease, int, pidfd, unsigned int, flags)
> > +{
> > +#ifdef CONFIG_MMU
> > +     struct mm_struct *mm = NULL;
> > +     struct task_struct *task;
> > +     struct task_struct *p;
> > +     unsigned int f_flags;
> > +     struct pid *pid;
> > +     long ret = 0;
> > +
> > +     if (flags)
> > +             return -EINVAL;
> > +
> > +     pid = pidfd_get_pid(pidfd, &f_flags);
> > +     if (IS_ERR(pid))
> > +             return PTR_ERR(pid);
> > +
> > +     task = get_pid_task(pid, PIDTYPE_PID);
> > +     if (!task) {
> > +             ret = -ESRCH;
> > +             goto put_pid;
> > +     }
> > +
> > +     /*
> > +      * If the task is dying and in the process of releasing its memory
> > +      * then get its mm.
> > +      */
> > +     p = find_lock_task_mm(task);
> > +     if (!p) {
> > +             ret = -ESRCH;
> > +             goto put_pid;
> > +     }
> > +     if (task != p) {
> > +             get_task_struct(p);
>
>
> Wouldn't we want to obtain the mm from p ? I thought that was the whole
> exercise of going via find_lock_task_mm().

Yes, that's what we do after checking task_will_free_mem().
task_will_free_mem() requires us to hold task_lock and
find_lock_task_mm() achieves that ensuring that mm is still valid, but
it might return a task other than the original one. That's why we do
this dance with pinning the new task and unpinning the original one.
The same dance is performed in __oom_kill_process(). I was
contemplating adding a parameter to find_lock_task_mm() to request
this unpin/pin be done within that function but then decided to keep
it simple for now.
Did I address your question or did I misunderstand it?

>
> --
> Thanks,
>
> David / dhildenb
>
> --
> To unsubscribe from this group and stop receiving emails from it, send an email to kernel-team+unsubscribe@android.com.
>

Re: [PATCH v7 1/2] mm: introduce process_mrelease system call

[David Hildenbrand]

On 05.08.21 19:49, Suren Baghdasaryan wrote:
> On Thu, Aug 5, 2021 at 10:29 AM David Hildenbrand <david@redhat.com> wrote:
>>
>> On 05.08.21 19:08, Suren Baghdasaryan wrote:
>>> In modern systems it's not unusual to have a system component monitoring
>>> memory conditions of the system and tasked with keeping system memory
>>> pressure under control. One way to accomplish that is to kill
>>> non-essential processes to free up memory for more important ones.
>>> Examples of this are Facebook's OOM killer daemon called oomd and
>>> Android's low memory killer daemon called lmkd.
>>> For such system component it's important to be able to free memory
>>> quickly and efficiently. Unfortunately the time process takes to free
>>> up its memory after receiving a SIGKILL might vary based on the state
>>> of the process (uninterruptible sleep), size and OPP level of the core
>>> the process is running. A mechanism to free resources of the target
>>> process in a more predictable way would improve system's ability to
>>> control its memory pressure.
>>> Introduce process_mrelease system call that releases memory of a dying
>>> process from the context of the caller. This way the memory is freed in
>>> a more controllable way with CPU affinity and priority of the caller.
>>> The workload of freeing the memory will also be charged to the caller.
>>> The operation is allowed only on a dying process.
>>>
>>> After previous discussions [1, 2, 3] the decision was made [4] to introduce
>>> a dedicated system call to cover this use case.
>>>
>>> The API is as follows,
>>>
>>>             int process_mrelease(int pidfd, unsigned int flags);
>>>
>>>           DESCRIPTION
>>>             The process_mrelease() system call is used to free the memory of
>>>             an exiting process.
>>>
>>>             The pidfd selects the process referred to by the PID file
>>>             descriptor.
>>>             (See pidfd_open(2) for further information)
>>>
>>>             The flags argument is reserved for future use; currently, this
>>>             argument must be specified as 0.
>>>
>>>           RETURN VALUE
>>>             On success, process_mrelease() returns 0. On error, -1 is
>>>             returned and errno is set to indicate the error.
>>>
>>>           ERRORS
>>>             EBADF  pidfd is not a valid PID file descriptor.
>>>
>>>             EAGAIN Failed to release part of the address space.
>>>
>>>             EINTR  The call was interrupted by a signal; see signal(7).
>>>
>>>             EINVAL flags is not 0.
>>>
>>>             EINVAL The memory of the task cannot be released because the
>>>                    process is not exiting, the address space is shared
>>>                    with another live process or there is a core dump in
>>>                    progress.
>>>
>>>             ENOSYS This system call is not supported, for example, without
>>>                    MMU support built into Linux.
>>>
>>>             ESRCH  The target process does not exist (i.e., it has terminated
>>>                    and been waited on).
>>>
>>> [1] https://lore.kernel.org/lkml/20190411014353.113252-3-surenb@google.com/
>>> [2] https://lore.kernel.org/linux-api/20201113173448.1863419-1-surenb@google.com/
>>> [3] https://lore.kernel.org/linux-api/20201124053943.1684874-3-surenb@google.com/
>>> [4] https://lore.kernel.org/linux-api/20201223075712.GA4719@lst.de/
>>>
>>> Signed-off-by: Suren Baghdasaryan <surenb@google.com>
>>> ---
>>> changes in v7:
>>> - Fixed pidfd_open misspelling, per Andrew Morton
>>> - Fixed wrong task pinning after find_lock_task_mm() issue, per Michal Hocko
>>> - Moved MMF_OOM_SKIP check before task_will_free_mem(), per Michal Hocko
>>>
>>>    mm/oom_kill.c | 73 +++++++++++++++++++++++++++++++++++++++++++++++++++
>>>    1 file changed, 73 insertions(+)
>>>
>>> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
>>> index c729a4c4a1ac..a4d917b43c73 100644
>>> --- a/mm/oom_kill.c
>>> +++ b/mm/oom_kill.c
>>> @@ -28,6 +28,7 @@
>>>    #include <linux/sched/task.h>
>>>    #include <linux/sched/debug.h>
>>>    #include <linux/swap.h>
>>> +#include <linux/syscalls.h>
>>>    #include <linux/timex.h>
>>>    #include <linux/jiffies.h>
>>>    #include <linux/cpuset.h>
>>> @@ -1141,3 +1142,75 @@ void pagefault_out_of_memory(void)
>>>        out_of_memory(&oc);
>>>        mutex_unlock(&oom_lock);
>>>    }
>>> +
>>> +SYSCALL_DEFINE2(process_mrelease, int, pidfd, unsigned int, flags)
>>> +{
>>> +#ifdef CONFIG_MMU
>>> +     struct mm_struct *mm = NULL;
>>> +     struct task_struct *task;
>>> +     struct task_struct *p;
>>> +     unsigned int f_flags;
>>> +     struct pid *pid;
>>> +     long ret = 0;
>>> +
>>> +     if (flags)
>>> +             return -EINVAL;
>>> +
>>> +     pid = pidfd_get_pid(pidfd, &f_flags);
>>> +     if (IS_ERR(pid))
>>> +             return PTR_ERR(pid);
>>> +
>>> +     task = get_pid_task(pid, PIDTYPE_PID);
>>> +     if (!task) {
>>> +             ret = -ESRCH;
>>> +             goto put_pid;
>>> +     }
>>> +
>>> +     /*
>>> +      * If the task is dying and in the process of releasing its memory
>>> +      * then get its mm.
>>> +      */
>>> +     p = find_lock_task_mm(task);
>>> +     if (!p) {
>>> +             ret = -ESRCH;
>>> +             goto put_pid;
>>> +     }
>>> +     if (task != p) {
>>> +             get_task_struct(p);
>>
>>
>> Wouldn't we want to obtain the mm from p ? I thought that was the whole
>> exercise of going via find_lock_task_mm().
> 
> Yes, that's what we do after checking task_will_free_mem().
> task_will_free_mem() requires us to hold task_lock and
> find_lock_task_mm() achieves that ensuring that mm is still valid, but
> it might return a task other than the original one. That's why we do
> this dance with pinning the new task and unpinning the original one.
> The same dance is performed in __oom_kill_process(). I was
> contemplating adding a parameter to find_lock_task_mm() to request
> this unpin/pin be done within that function but then decided to keep
> it simple for now.
> Did I address your question or did I misunderstand it?

Excuse my tired eyes, I missed the "task = p;"

Feel free to carry my ack along, even if there are minor changes.


-- 
Thanks,

David / dhildenb

Re: [PATCH v7 1/2] mm: introduce process_mrelease system call

[Shakeel Butt]

On Thu, Aug 5, 2021 at 10:50 AM Suren Baghdasaryan <surenb@google.com> wrote:
>
> On Thu, Aug 5, 2021 at 10:29 AM David Hildenbrand <david@redhat.com> wrote:
> >
> > On 05.08.21 19:08, Suren Baghdasaryan wrote:
> > > In modern systems it's not unusual to have a system component monitoring
> > > memory conditions of the system and tasked with keeping system memory
> > > pressure under control. One way to accomplish that is to kill
> > > non-essential processes to free up memory for more important ones.
> > > Examples of this are Facebook's OOM killer daemon called oomd and
> > > Android's low memory killer daemon called lmkd.
> > > For such system component it's important to be able to free memory
> > > quickly and efficiently. Unfortunately the time process takes to free
> > > up its memory after receiving a SIGKILL might vary based on the state
> > > of the process (uninterruptible sleep), size and OPP level of the core
> > > the process is running. A mechanism to free resources of the target
> > > process in a more predictable way would improve system's ability to
> > > control its memory pressure.
> > > Introduce process_mrelease system call that releases memory of a dying
> > > process from the context of the caller. This way the memory is freed in
> > > a more controllable way with CPU affinity and priority of the caller.
> > > The workload of freeing the memory will also be charged to the caller.
> > > The operation is allowed only on a dying process.
> > >
> > > After previous discussions [1, 2, 3] the decision was made [4] to introduce
> > > a dedicated system call to cover this use case.
> > >
> > > The API is as follows,
> > >
> > >            int process_mrelease(int pidfd, unsigned int flags);
> > >
> > >          DESCRIPTION
> > >            The process_mrelease() system call is used to free the memory of
> > >            an exiting process.
> > >
> > >            The pidfd selects the process referred to by the PID file
> > >            descriptor.
> > >            (See pidfd_open(2) for further information)
> > >
> > >            The flags argument is reserved for future use; currently, this
> > >            argument must be specified as 0.
> > >
> > >          RETURN VALUE
> > >            On success, process_mrelease() returns 0. On error, -1 is
> > >            returned and errno is set to indicate the error.
> > >
> > >          ERRORS
> > >            EBADF  pidfd is not a valid PID file descriptor.
> > >
> > >            EAGAIN Failed to release part of the address space.
> > >
> > >            EINTR  The call was interrupted by a signal; see signal(7).
> > >
> > >            EINVAL flags is not 0.
> > >
> > >            EINVAL The memory of the task cannot be released because the
> > >                   process is not exiting, the address space is shared
> > >                   with another live process or there is a core dump in
> > >                   progress.
> > >
> > >            ENOSYS This system call is not supported, for example, without
> > >                   MMU support built into Linux.
> > >
> > >            ESRCH  The target process does not exist (i.e., it has terminated
> > >                   and been waited on).
> > >
> > > [1] https://lore.kernel.org/lkml/20190411014353.113252-3-surenb@google.com/
> > > [2] https://lore.kernel.org/linux-api/20201113173448.1863419-1-surenb@google.com/
> > > [3] https://lore.kernel.org/linux-api/20201124053943.1684874-3-surenb@google.com/
> > > [4] https://lore.kernel.org/linux-api/20201223075712.GA4719@lst.de/
> > >
> > > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > > ---
> > > changes in v7:
> > > - Fixed pidfd_open misspelling, per Andrew Morton
> > > - Fixed wrong task pinning after find_lock_task_mm() issue, per Michal Hocko
> > > - Moved MMF_OOM_SKIP check before task_will_free_mem(), per Michal Hocko
> > >
> > >   mm/oom_kill.c | 73 +++++++++++++++++++++++++++++++++++++++++++++++++++
> > >   1 file changed, 73 insertions(+)
> > >
> > > diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> > > index c729a4c4a1ac..a4d917b43c73 100644
> > > --- a/mm/oom_kill.c
> > > +++ b/mm/oom_kill.c
> > > @@ -28,6 +28,7 @@
> > >   #include <linux/sched/task.h>
> > >   #include <linux/sched/debug.h>
> > >   #include <linux/swap.h>
> > > +#include <linux/syscalls.h>
> > >   #include <linux/timex.h>
> > >   #include <linux/jiffies.h>
> > >   #include <linux/cpuset.h>
> > > @@ -1141,3 +1142,75 @@ void pagefault_out_of_memory(void)
> > >       out_of_memory(&oc);
> > >       mutex_unlock(&oom_lock);
> > >   }
> > > +
> > > +SYSCALL_DEFINE2(process_mrelease, int, pidfd, unsigned int, flags)
> > > +{
> > > +#ifdef CONFIG_MMU
> > > +     struct mm_struct *mm = NULL;
> > > +     struct task_struct *task;
> > > +     struct task_struct *p;
> > > +     unsigned int f_flags;
> > > +     struct pid *pid;
> > > +     long ret = 0;
> > > +
> > > +     if (flags)
> > > +             return -EINVAL;
> > > +
> > > +     pid = pidfd_get_pid(pidfd, &f_flags);
> > > +     if (IS_ERR(pid))
> > > +             return PTR_ERR(pid);
> > > +
> > > +     task = get_pid_task(pid, PIDTYPE_PID);
> > > +     if (!task) {
> > > +             ret = -ESRCH;
> > > +             goto put_pid;
> > > +     }
> > > +
> > > +     /*
> > > +      * If the task is dying and in the process of releasing its memory
> > > +      * then get its mm.
> > > +      */
> > > +     p = find_lock_task_mm(task);
> > > +     if (!p) {
> > > +             ret = -ESRCH;
> > > +             goto put_pid;
> > > +     }
> > > +     if (task != p) {
> > > +             get_task_struct(p);
> >
> >
> > Wouldn't we want to obtain the mm from p ? I thought that was the whole
> > exercise of going via find_lock_task_mm().
>
> Yes, that's what we do after checking task_will_free_mem().
> task_will_free_mem() requires us to hold task_lock and
> find_lock_task_mm() achieves that ensuring that mm is still valid, but
> it might return a task other than the original one. That's why we do
> this dance with pinning the new task and unpinning the original one.
> The same dance is performed in __oom_kill_process(). I was
> contemplating adding a parameter to find_lock_task_mm() to request
> this unpin/pin be done within that function but then decided to keep
> it simple for now.
> Did I address your question or did I misunderstand it?
>

One question I have is why mmget() and not mmgrab()? I see mmgrab() in
oom_kill.c.

Re: [PATCH v7 1/2] mm: introduce process_mrelease system call

[Suren Baghdasaryan]

On Thu, Aug 5, 2021 at 10:56 AM Shakeel Butt <shakeelb@google.com> wrote:
>
> On Thu, Aug 5, 2021 at 10:50 AM Suren Baghdasaryan <surenb@google.com> wrote:
> >
> > On Thu, Aug 5, 2021 at 10:29 AM David Hildenbrand <david@redhat.com> wrote:
> > >
> > > On 05.08.21 19:08, Suren Baghdasaryan wrote:
> > > > In modern systems it's not unusual to have a system component monitoring
> > > > memory conditions of the system and tasked with keeping system memory
> > > > pressure under control. One way to accomplish that is to kill
> > > > non-essential processes to free up memory for more important ones.
> > > > Examples of this are Facebook's OOM killer daemon called oomd and
> > > > Android's low memory killer daemon called lmkd.
> > > > For such system component it's important to be able to free memory
> > > > quickly and efficiently. Unfortunately the time process takes to free
> > > > up its memory after receiving a SIGKILL might vary based on the state
> > > > of the process (uninterruptible sleep), size and OPP level of the core
> > > > the process is running. A mechanism to free resources of the target
> > > > process in a more predictable way would improve system's ability to
> > > > control its memory pressure.
> > > > Introduce process_mrelease system call that releases memory of a dying
> > > > process from the context of the caller. This way the memory is freed in
> > > > a more controllable way with CPU affinity and priority of the caller.
> > > > The workload of freeing the memory will also be charged to the caller.
> > > > The operation is allowed only on a dying process.
> > > >
> > > > After previous discussions [1, 2, 3] the decision was made [4] to introduce
> > > > a dedicated system call to cover this use case.
> > > >
> > > > The API is as follows,
> > > >
> > > >            int process_mrelease(int pidfd, unsigned int flags);
> > > >
> > > >          DESCRIPTION
> > > >            The process_mrelease() system call is used to free the memory of
> > > >            an exiting process.
> > > >
> > > >            The pidfd selects the process referred to by the PID file
> > > >            descriptor.
> > > >            (See pidfd_open(2) for further information)
> > > >
> > > >            The flags argument is reserved for future use; currently, this
> > > >            argument must be specified as 0.
> > > >
> > > >          RETURN VALUE
> > > >            On success, process_mrelease() returns 0. On error, -1 is
> > > >            returned and errno is set to indicate the error.
> > > >
> > > >          ERRORS
> > > >            EBADF  pidfd is not a valid PID file descriptor.
> > > >
> > > >            EAGAIN Failed to release part of the address space.
> > > >
> > > >            EINTR  The call was interrupted by a signal; see signal(7).
> > > >
> > > >            EINVAL flags is not 0.
> > > >
> > > >            EINVAL The memory of the task cannot be released because the
> > > >                   process is not exiting, the address space is shared
> > > >                   with another live process or there is a core dump in
> > > >                   progress.
> > > >
> > > >            ENOSYS This system call is not supported, for example, without
> > > >                   MMU support built into Linux.
> > > >
> > > >            ESRCH  The target process does not exist (i.e., it has terminated
> > > >                   and been waited on).
> > > >
> > > > [1] https://lore.kernel.org/lkml/20190411014353.113252-3-surenb@google.com/
> > > > [2] https://lore.kernel.org/linux-api/20201113173448.1863419-1-surenb@google.com/
> > > > [3] https://lore.kernel.org/linux-api/20201124053943.1684874-3-surenb@google.com/
> > > > [4] https://lore.kernel.org/linux-api/20201223075712.GA4719@lst.de/
> > > >
> > > > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > > > ---
> > > > changes in v7:
> > > > - Fixed pidfd_open misspelling, per Andrew Morton
> > > > - Fixed wrong task pinning after find_lock_task_mm() issue, per Michal Hocko
> > > > - Moved MMF_OOM_SKIP check before task_will_free_mem(), per Michal Hocko
> > > >
> > > >   mm/oom_kill.c | 73 +++++++++++++++++++++++++++++++++++++++++++++++++++
> > > >   1 file changed, 73 insertions(+)
> > > >
> > > > diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> > > > index c729a4c4a1ac..a4d917b43c73 100644
> > > > --- a/mm/oom_kill.c
> > > > +++ b/mm/oom_kill.c
> > > > @@ -28,6 +28,7 @@
> > > >   #include <linux/sched/task.h>
> > > >   #include <linux/sched/debug.h>
> > > >   #include <linux/swap.h>
> > > > +#include <linux/syscalls.h>
> > > >   #include <linux/timex.h>
> > > >   #include <linux/jiffies.h>
> > > >   #include <linux/cpuset.h>
> > > > @@ -1141,3 +1142,75 @@ void pagefault_out_of_memory(void)
> > > >       out_of_memory(&oc);
> > > >       mutex_unlock(&oom_lock);
> > > >   }
> > > > +
> > > > +SYSCALL_DEFINE2(process_mrelease, int, pidfd, unsigned int, flags)
> > > > +{
> > > > +#ifdef CONFIG_MMU
> > > > +     struct mm_struct *mm = NULL;
> > > > +     struct task_struct *task;
> > > > +     struct task_struct *p;
> > > > +     unsigned int f_flags;
> > > > +     struct pid *pid;
> > > > +     long ret = 0;
> > > > +
> > > > +     if (flags)
> > > > +             return -EINVAL;
> > > > +
> > > > +     pid = pidfd_get_pid(pidfd, &f_flags);
> > > > +     if (IS_ERR(pid))
> > > > +             return PTR_ERR(pid);
> > > > +
> > > > +     task = get_pid_task(pid, PIDTYPE_PID);
> > > > +     if (!task) {
> > > > +             ret = -ESRCH;
> > > > +             goto put_pid;
> > > > +     }
> > > > +
> > > > +     /*
> > > > +      * If the task is dying and in the process of releasing its memory
> > > > +      * then get its mm.
> > > > +      */
> > > > +     p = find_lock_task_mm(task);
> > > > +     if (!p) {
> > > > +             ret = -ESRCH;
> > > > +             goto put_pid;
> > > > +     }
> > > > +     if (task != p) {
> > > > +             get_task_struct(p);
> > >
> > >
> > > Wouldn't we want to obtain the mm from p ? I thought that was the whole
> > > exercise of going via find_lock_task_mm().
> >
> > Yes, that's what we do after checking task_will_free_mem().
> > task_will_free_mem() requires us to hold task_lock and
> > find_lock_task_mm() achieves that ensuring that mm is still valid, but
> > it might return a task other than the original one. That's why we do
> > this dance with pinning the new task and unpinning the original one.
> > The same dance is performed in __oom_kill_process(). I was
> > contemplating adding a parameter to find_lock_task_mm() to request
> > this unpin/pin be done within that function but then decided to keep
> > it simple for now.
> > Did I address your question or did I misunderstand it?
> >
>
> One question I have is why mmget() and not mmgrab()? I see mmgrab() in
> oom_kill.c.

You are likely right here. The caller's context probably can't be
considered a "real user" when reaping the mm. However, we take an
mmap_lock shortly after, so not sure if in practice there is much
difference.
Michal, WDYT?

Re: [PATCH v7 1/2] mm: introduce process_mrelease system call

[Michal Hocko]

On Thu 05-08-21 10:08:58, Suren Baghdasaryan wrote:
[...]
> +	/*
> +	 * If the task is dying and in the process of releasing its memory
> +	 * then get its mm.
> +	 */
> +	p = find_lock_task_mm(task);
> +	if (!p) {
> +		ret = -ESRCH;
> +		goto put_pid;
> +	}
> +	if (task != p) {
> +		get_task_struct(p);
> +		put_task_struct(task);
> +		task = p;
> +	}

Why do you need to take a reference to the p here? You are under
task_lock so this will not go away and you only need p to get your mm.

> +
> +	/* If the work has been done already, just exit with success */
> +	if (test_bit(MMF_OOM_SKIP, &task->mm->flags))
> +		goto put_task;

You want to release the task_lock

> +
> +	if (task_will_free_mem(task) && (task->flags & PF_KTHREAD) == 0) {

you want task_will_free_mem(p) and what is the point of the PF_KTHREAD
check?

> +		mm = task->mm;
> +		mmget(mm);

All you need is to make sure mm will not get released under your feet
once task_lock is released so mmgrab is the right thing to do here. The
address space can be torn down in parallel and that is OK and desirable.

I think you really want something like this:

	if (flags)
		return -EINVAL;
	
	pid = pidfd_get_pid(fd, &f_flags);
	if (IS_ERR(pid))
		return PTR_ERR(pid);
	task = get_pid_task(pid, PIDTYPE_PID);
	if (!task) {
		ret = -ESRCH;
		goto put_pid;
	}

	/*
	 * Make sure to chose a thread which still has a reference to mm
	 * during the group exit
	 */
	p = find_lock_task_mm(task);
	if (!p) {
		ret = -ESRCH;
		goto put_task;
	}

	mm = task->mm;
	mmgrab(mm);
	reap = true;
	/* If the work has been done already, just exit with success */
	if (test_bit(MMF_OOM_SKIP, &mm->flags)) {
		reap = false;
	} else if (!task_will_free_mem(p)) {
		reap = false;
		ret = -EINVAL;
	}
	task_unlock(p);

	if (!reap)
		goto dropmm;;

	/* Do the work*/


dropmm:
	mmdrop(mm);
put_task:
	put_task(task);
put_pid:
	put_pid(pid);

	return ret;

-- 
Michal Hocko
SUSE Labs

Re: [PATCH v7 1/2] mm: introduce process_mrelease system call

[Michal Hocko]

On Thu 05-08-21 11:37:06, Suren Baghdasaryan wrote:
[...]
> > One question I have is why mmget() and not mmgrab()? I see mmgrab() in
> > oom_kill.c.
> 
> You are likely right here. The caller's context probably can't be
> considered a "real user" when reaping the mm. However, we take an
> mmap_lock shortly after, so not sure if in practice there is much
> difference.
> Michal, WDYT?

As explained in other response. mmget is to pin address space to not go
away. You do not need that for this purpose. All you need is to pin mm
to not go away. Address space can be unmapped concurrently.
-- 
Michal Hocko
SUSE Labs

Re: [PATCH v7 1/2] mm: introduce process_mrelease system call

[Shakeel Butt]

On Thu, Aug 5, 2021 at 11:40 PM Michal Hocko <mhocko@suse.com> wrote:
>
[...]
> I think you really want something like this:
>
>         if (flags)
>                 return -EINVAL;
>
>         pid = pidfd_get_pid(fd, &f_flags);
>         if (IS_ERR(pid))
>                 return PTR_ERR(pid);
>         task = get_pid_task(pid, PIDTYPE_PID);
>         if (!task) {
>                 ret = -ESRCH;
>                 goto put_pid;
>         }
>
>         /*
>          * Make sure to chose a thread which still has a reference to mm
>          * during the group exit
>          */
>         p = find_lock_task_mm(task);
>         if (!p) {
>                 ret = -ESRCH;
>                 goto put_task;
>         }
>
>         mm = task->mm;

mm = p->mm;

>         mmgrab(mm);
>         reap = true;
>         /* If the work has been done already, just exit with success */
>         if (test_bit(MMF_OOM_SKIP, &mm->flags)) {
>                 reap = false;
>         } else if (!task_will_free_mem(p)) {
>                 reap = false;
>                 ret = -EINVAL;
>         }
>         task_unlock(p);
>
>         if (!reap)
>                 goto dropmm;;
>
>         /* Do the work*/
>
>
> dropmm:
>         mmdrop(mm);
> put_task:
>         put_task(task);
> put_pid:
>         put_pid(pid);
>
>         return ret;
>
> --
> Michal Hocko
> SUSE Labs

Re: [PATCH v7 1/2] mm: introduce process_mrelease system call

[Michal Hocko]

On Fri 06-08-21 02:23:17, Shakeel Butt wrote:
> On Thu, Aug 5, 2021 at 11:40 PM Michal Hocko <mhocko@suse.com> wrote:
> >
> [...]
> > I think you really want something like this:
> >
> >         if (flags)
> >                 return -EINVAL;
> >
> >         pid = pidfd_get_pid(fd, &f_flags);
> >         if (IS_ERR(pid))
> >                 return PTR_ERR(pid);
> >         task = get_pid_task(pid, PIDTYPE_PID);
> >         if (!task) {
> >                 ret = -ESRCH;
> >                 goto put_pid;
> >         }
> >
> >         /*
> >          * Make sure to chose a thread which still has a reference to mm
> >          * during the group exit
> >          */
> >         p = find_lock_task_mm(task);
> >         if (!p) {
> >                 ret = -ESRCH;
> >                 goto put_task;
> >         }
> >
> >         mm = task->mm;
> 
> mm = p->mm;

right. Thanks!

-- 
Michal Hocko
SUSE Labs

Re: [PATCH v7 1/2] mm: introduce process_mrelease system call

[Suren Baghdasaryan]

On Thu, Aug 5, 2021 at 11:40 PM Michal Hocko <mhocko@suse.com> wrote:
>
> On Thu 05-08-21 10:08:58, Suren Baghdasaryan wrote:
> [...]
> > +     /*
> > +      * If the task is dying and in the process of releasing its memory
> > +      * then get its mm.
> > +      */
> > +     p = find_lock_task_mm(task);
> > +     if (!p) {
> > +             ret = -ESRCH;
> > +             goto put_pid;
> > +     }
> > +     if (task != p) {
> > +             get_task_struct(p);
> > +             put_task_struct(task);
> > +             task = p;
> > +     }
>
> Why do you need to take a reference to the p here? You are under
> task_lock so this will not go away and you only need p to get your mm.

True.

>
> > +
> > +     /* If the work has been done already, just exit with success */
> > +     if (test_bit(MMF_OOM_SKIP, &task->mm->flags))
> > +             goto put_task;
>
> You want to release the task_lock

Missed it again :(

>
> > +
> > +     if (task_will_free_mem(task) && (task->flags & PF_KTHREAD) == 0) {
>
> you want task_will_free_mem(p) and what is the point of the PF_KTHREAD
> check?

Yeah, looks like task_will_free_mem() covers that case already.

>
> > +             mm = task->mm;
> > +             mmget(mm);
>
> All you need is to make sure mm will not get released under your feet
> once task_lock is released so mmgrab is the right thing to do here. The
> address space can be torn down in parallel and that is OK and desirable.
>
> I think you really want something like this:
>
>         if (flags)
>                 return -EINVAL;
>
>         pid = pidfd_get_pid(fd, &f_flags);
>         if (IS_ERR(pid))
>                 return PTR_ERR(pid);
>         task = get_pid_task(pid, PIDTYPE_PID);
>         if (!task) {
>                 ret = -ESRCH;
>                 goto put_pid;
>         }
>
>         /*
>          * Make sure to chose a thread which still has a reference to mm
>          * during the group exit
>          */
>         p = find_lock_task_mm(task);
>         if (!p) {
>                 ret = -ESRCH;
>                 goto put_task;
>         }
>
>         mm = task->mm;
>         mmgrab(mm);
>         reap = true;
>         /* If the work has been done already, just exit with success */
>         if (test_bit(MMF_OOM_SKIP, &mm->flags)) {
>                 reap = false;
>         } else if (!task_will_free_mem(p)) {
>                 reap = false;
>                 ret = -EINVAL;
>         }
>         task_unlock(p);
>
>         if (!reap)
>                 goto dropmm;;
>
>         /* Do the work*/
>
>
> dropmm:
>         mmdrop(mm);
> put_task:
>         put_task(task);
> put_pid:
>         put_pid(pid);
>
>         return ret;
>

This is indeed simpler to follow. I'll adopt your version. Thanks!

> --
> Michal Hocko
> SUSE Labs

Re: [PATCH v7 1/2] mm: introduce process_mrelease system call

[Suren Baghdasaryan]

On Fri, Aug 6, 2021 at 9:07 AM Suren Baghdasaryan <surenb@google.com> wrote:
>
> On Thu, Aug 5, 2021 at 11:40 PM Michal Hocko <mhocko@suse.com> wrote:
> >
> > On Thu 05-08-21 10:08:58, Suren Baghdasaryan wrote:
> > [...]
> > > +     /*
> > > +      * If the task is dying and in the process of releasing its memory
> > > +      * then get its mm.
> > > +      */
> > > +     p = find_lock_task_mm(task);
> > > +     if (!p) {
> > > +             ret = -ESRCH;
> > > +             goto put_pid;
> > > +     }
> > > +     if (task != p) {
> > > +             get_task_struct(p);
> > > +             put_task_struct(task);
> > > +             task = p;
> > > +     }
> >
> > Why do you need to take a reference to the p here? You are under
> > task_lock so this will not go away and you only need p to get your mm.
>
> True.
>
> >
> > > +
> > > +     /* If the work has been done already, just exit with success */
> > > +     if (test_bit(MMF_OOM_SKIP, &task->mm->flags))
> > > +             goto put_task;
> >
> > You want to release the task_lock
>
> Missed it again :(
>
> >
> > > +
> > > +     if (task_will_free_mem(task) && (task->flags & PF_KTHREAD) == 0) {
> >
> > you want task_will_free_mem(p) and what is the point of the PF_KTHREAD
> > check?
>
> Yeah, looks like task_will_free_mem() covers that case already.
>
> >
> > > +             mm = task->mm;
> > > +             mmget(mm);
> >
> > All you need is to make sure mm will not get released under your feet
> > once task_lock is released so mmgrab is the right thing to do here. The
> > address space can be torn down in parallel and that is OK and desirable.
> >
> > I think you really want something like this:
> >
> >         if (flags)
> >                 return -EINVAL;
> >
> >         pid = pidfd_get_pid(fd, &f_flags);
> >         if (IS_ERR(pid))
> >                 return PTR_ERR(pid);
> >         task = get_pid_task(pid, PIDTYPE_PID);
> >         if (!task) {
> >                 ret = -ESRCH;
> >                 goto put_pid;
> >         }
> >
> >         /*
> >          * Make sure to chose a thread which still has a reference to mm
> >          * during the group exit
> >          */
> >         p = find_lock_task_mm(task);
> >         if (!p) {
> >                 ret = -ESRCH;
> >                 goto put_task;
> >         }
> >
> >         mm = task->mm;
> >         mmgrab(mm);
> >         reap = true;
> >         /* If the work has been done already, just exit with success */
> >         if (test_bit(MMF_OOM_SKIP, &mm->flags)) {
> >                 reap = false;
> >         } else if (!task_will_free_mem(p)) {
> >                 reap = false;
> >                 ret = -EINVAL;
> >         }
> >         task_unlock(p);
> >
> >         if (!reap)
> >                 goto dropmm;;
> >
> >         /* Do the work*/
> >
> >
> > dropmm:
> >         mmdrop(mm);
> > put_task:
> >         put_task(task);
> > put_pid:
> >         put_pid(pid);
> >
> >         return ret;
> >
>
> This is indeed simpler to follow. I'll adopt your version. Thanks!

v8 is posted at https://lore.kernel.org/patchwork/patch/1473697/
Testing shows performance improvement from replacing mmget with mmgrab.

>
> > --
> > Michal Hocko
> > SUSE Labs