[patch v2] mm, oom: prevent soft lockup on memcg oom for UP systems

[David Rientjes <rientjes@google.com>]

When a process is oom killed as a result of memcg limits and the victim
is waiting to exit, nothing ends up actually yielding the processor back
to the victim on UP systems with preemption disabled.  Instead, the
charging process simply loops in memcg reclaim and eventually soft
lockups.

Memory cgroup out of memory: Killed process 808 (repro) total-vm:41944kB, 
anon-rss:35344kB, file-rss:504kB, shmem-rss:0kB, UID:0 pgtables:108kB 
oom_score_adj:0
watchdog: BUG: soft lockup - CPU#0 stuck for 23s! [repro:806]
CPU: 0 PID: 806 Comm: repro Not tainted 5.6.0-rc5+ #136
RIP: 0010:shrink_lruvec+0x4e9/0xa40
...
Call Trace:
 shrink_node+0x40d/0x7d0
 do_try_to_free_pages+0x13f/0x470
 try_to_free_mem_cgroup_pages+0x16d/0x230
 try_charge+0x247/0xac0
 mem_cgroup_try_charge+0x10a/0x220
 mem_cgroup_try_charge_delay+0x1e/0x40
 handle_mm_fault+0xdf2/0x15f0
 do_user_addr_fault+0x21f/0x420
 page_fault+0x2f/0x40

Make sure that once the oom killer has been called that we forcibly yield 
if current is not the chosen victim regardless of priority to allow for 
memory freeing.  The same situation can theoretically occur in the page 
allocator, so do this after dropping oom_lock there as well.

Suggested-by: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Tested-by: Robert Kolchmeyer <rkolchmeyer@google.com>
Cc: stable@vger.kernel.org
Signed-off-by: David Rientjes <rientjes@google.com>
---
 mm/memcontrol.c | 2 ++
 mm/page_alloc.c | 2 ++
 2 files changed, 4 insertions(+)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1576,6 +1576,8 @@ static bool mem_cgroup_out_of_memory(struct mem_cgroup *memcg, gfp_t gfp_mask,
 	 */
 	ret = should_force_charge() || out_of_memory(&oc);
 	mutex_unlock(&oom_lock);
+	if (!fatal_signal_pending(current))
+		schedule_timeout_killable(1);
 	return ret;
 }
 
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3861,6 +3861,8 @@ __alloc_pages_may_oom(gfp_t gfp_mask, unsigned int order,
 	}
 out:
 	mutex_unlock(&oom_lock);
+	if (!fatal_signal_pending(current))
+		schedule_timeout_killable(1);
 	return page;
 }