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From: Peter Oskolkov <posk@google.com>
To: Jann Horn <jannh@google.com>
Cc: Peter Oskolkov <posk@posk.io>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	linux-kernel@vger.kernel.org, linux-api@vger.kernel.org,
	Paul Turner <pjt@google.com>, Ben Segall <bsegall@google.com>,
	Andrei Vagin <avagin@google.com>,
	Thierry Delisle <tdelisle@uwaterloo.ca>
Subject: Re: [PATCH 2/4 v0.5] sched/umcg: RFC: add userspace atomic helpers
Date: Thu, 9 Sep 2021 12:06:58 -0700	[thread overview]
Message-ID: <CAPNVh5eaW7r_Nv-wHEyxQiFkXngmONwPyZSFvtTEhk3TxJ+iMA@mail.gmail.com> (raw)
In-Reply-To: <CAG48ez2LyLNkH4iVbeKJUuH=oh57WECkKYCW+G9mtheoh7Fsvg@mail.gmail.com>

On Wed, Sep 8, 2021 at 4:39 PM Jann Horn <jannh@google.com> wrote:

Thanks a lot for the reviews, Jann!

I understand how to address most of your comments. However, one issue
I'm not sure what to do about:

[...]

> If this function is not allowed to sleep, as the comment says...

[...]

> ... then I'm pretty sure you can't call fix_pagefault() here, which
> acquires the mmap semaphore (which may involve sleeping) and then goes
> through the pagefault handling path (which can also sleep for various
> reasons, like allocating memory for pagetables, loading pages from
> disk / NFS / FUSE, and so on).

<quote from peterz@ from
https://lore.kernel.org/lkml/20210609125435.GA68187@worktop.programming.kicks-ass.net/>:
  So a PF_UMCG_WORKER would be added to sched_submit_work()'s PF_*_WORKER
  path to capture these tasks blocking. The umcg_sleeping() hook added
  there would:

    put_user(BLOCKED, umcg_task->umcg_status);
    ...
</quote>

Which is basically what I am doing here: in sched_submit_work() I need
to read/write to userspace; and we cannot sleep in
sched_submit_work(), I believe.

If you are right that it is impossible to deal with pagefaults from
within non-sleepable contexts, I see two options:

Option 1: as you suggest, pin pages holding struct umcg_task in sys_umcg_ctl;

or

Option 2: add more umcg-related kernel state to task_struct so that
reading/writing to userspace is not necessary in sched_submit_work().

The first option sounds much better from the code simplicity point of
view, but I'm not sure if it is a viable approach, i.e. I'm afraid
we'll get a hard NACK here, as a non-privileged process will be able
to force the kernel to pin a page per task/thread. We may get around
it by first pinning a limited number of pages, then having the
userspace allocate structs umcg_task on those pages, so that a pinned
page would cover more than a single task/thread. And have a sysctl
that limits the number of pinned pages per MM.

Peter Z., could you, please, comment here? Do you think pinning pages
to hold structs umcg_task is acceptable?

[...]

  parent reply	other threads:[~2021-09-09 19:07 UTC|newest]

Thread overview: 27+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-08 18:49 [PATCH 0/4 v0.5] sched/umcg: RFC UMCG patchset Peter Oskolkov
2021-09-08 18:49 ` [PATCH 1/4 v0.5] sched/umcg: add WF_CURRENT_CPU and externise ttwu Peter Oskolkov
2021-09-08 18:49 ` [PATCH 2/4 v0.5] sched/umcg: RFC: add userspace atomic helpers Peter Oskolkov
2021-09-08 23:38   ` Jann Horn
2021-09-09  1:16     ` Jann Horn
2021-09-09 19:06     ` Peter Oskolkov [this message]
2021-09-09 21:20       ` Jann Horn
2021-09-09 22:09         ` Peter Oskolkov
2021-09-09 23:13           ` Jann Horn
2021-09-14 16:52         ` Andy Lutomirski
2021-09-14 18:11           ` Peter Zijlstra
2021-09-14 18:40             ` Andy Lutomirski
2021-09-15 15:42               ` Peter Zijlstra
2021-09-15 16:50                 ` Andy Lutomirski
2021-09-15 19:10                   ` Peter Zijlstra
2021-09-14  8:07       ` Peter Zijlstra
2021-09-14 16:29         ` Peter Oskolkov
2021-09-14 18:04           ` Peter Zijlstra
2021-09-14 18:15             ` Peter Zijlstra
2021-09-14 18:29             ` Peter Oskolkov
2021-09-14 18:48               ` Peter Oskolkov
2021-09-08 18:49 ` [PATCH 3/4 v0.5] sched/umcg: RFC: implement UMCG syscalls Peter Oskolkov
2021-09-09  1:39   ` Jann Horn
2021-09-14 16:51     ` Peter Oskolkov
2021-09-08 18:49 ` [PATCH 4/4 v0.5] sched/umcg: add Documentation/userspace-api/umcg.rst Peter Oskolkov
2021-09-14 16:35   ` Tao Zhou
2021-09-14 16:57     ` Peter Oskolkov

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