linux-kernel - [PATCH 2/2] mm/oom_kill: allow process_mrelease to run under mmap

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Message-Id: <20211116215715.645231-2-surenb@google.com>
Date:   Tue, 16 Nov 2021 13:57:15 -0800
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     akpm@...ux-foundation.org
Cc:     mhocko@...nel.org, mhocko@...e.com, rientjes@...gle.com,
        willy@...radead.org, hannes@...xchg.org, guro@...com,
        riel@...riel.com, minchan@...nel.org, kirill@...temov.name,
        aarcange@...hat.com, christian@...uner.io, hch@...radead.org,
        oleg@...hat.com, david@...hat.com, jannh@...gle.com,
        shakeelb@...gle.com, luto@...nel.org, christian.brauner@...ntu.com,
        fweimer@...hat.com, jengelh@...i.de, timmurray@...gle.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        kernel-team@...roid.com, surenb@...gle.com
Subject: [PATCH 2/2] mm/oom_kill: allow process_mrelease to run under
 mmap_lock protection

With exit_mmap holding mmap_write_lock during free_pgtables call,
process_mrelease does not need to elevate mm->mm_users in order to
prevent exit_mmap from destrying pagetables while __oom_reap_task_mm
is walking the VMA tree. The change prevents process_mrelease from
calling the last mmput, which can lead to waiting for IO completion
in exit_aio.

Fixes: 337546e83fc7 ("mm/oom_kill.c: prevent a race between process_mrelease and exit_mmap")
Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
---
 mm/oom_kill.c | 27 +++++++++++++++------------
 1 file changed, 15 insertions(+), 12 deletions(-)

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 1ddabefcfb5a..67780386f478 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -1169,15 +1169,15 @@ SYSCALL_DEFINE2(process_mrelease, int, pidfd, unsigned int, flags)
 		goto put_task;
 	}
 
-	if (mmget_not_zero(p->mm)) {
-		mm = p->mm;
-		if (task_will_free_mem(p))
-			reap = true;
-		else {
-			/* Error only if the work has not been done already */
-			if (!test_bit(MMF_OOM_SKIP, &mm->flags))
-				ret = -EINVAL;
-		}
+	mm = p->mm;
+	mmgrab(mm);
+
+	if (task_will_free_mem(p))
+		reap = true;
+	else {
+		/* Error only if the work has not been done already */
+		if (!test_bit(MMF_OOM_SKIP, &mm->flags))
+			ret = -EINVAL;
 	}
 	task_unlock(p);
 
@@ -1188,13 +1188,16 @@ SYSCALL_DEFINE2(process_mrelease, int, pidfd, unsigned int, flags)
 		ret = -EINTR;
 		goto drop_mm;
 	}
-	if (!__oom_reap_task_mm(mm))
+	/*
+	 * Check MMF_OOM_SKIP again under mmap_read_lock protection to ensure
+	 * possible change in exit_mmap is seen
+	 */
+	if (!test_bit(MMF_OOM_SKIP, &mm->flags) && !__oom_reap_task_mm(mm))
 		ret = -EAGAIN;
 	mmap_read_unlock(mm);
 
 drop_mm:
-	if (mm)
-		mmput(mm);
+	mmdrop(mm);
 put_task:
 	put_task_struct(task);
 	return ret;
-- 
2.34.0.rc1.387.gb447b232ab-goog