Date:   Thu, 9 Dec 2021 10:12:10 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     Suren Baghdasaryan <surenb@google.com>
Cc:     akpm@linux-foundation.org, rientjes@google.com,
        willy@infradead.org, hannes@cmpxchg.org, guro@fb.com,
        riel@surriel.com, minchan@kernel.org, kirill@shutemov.name,
        aarcange@redhat.com, christian@brauner.io, hch@infradead.org,
        oleg@redhat.com, david@redhat.com, jannh@google.com,
        shakeelb@google.com, luto@kernel.org, christian.brauner@ubuntu.com,
        fweimer@redhat.com, jengelh@inai.de, timmurray@google.com,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        kernel-team@android.com
Subject: [PATCH 4/3] mm: drop MMF_OOM_SKIP from exit_mmap
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Do we want this on top?
----
From 58b04ae6dc97b0105ea2651daca55cf2386f69b4 Mon Sep 17 00:00:00 2001
From: Michal Hocko <mhocko@suse.com>
Date: Thu, 9 Dec 2021 10:07:51 +0100
Subject: [PATCH] mm: drop MMF_OOM_SKIP from exit_mmap

MMF_OOM_SKIP used to play a synchronization role between exit_mmap and
oom repear in the past. Since the exclusive mmap_sem is held in
exit_mmap to cover all destructive operations the flag synchronization
is not needed anymore and we can safely drop it. Just make sure that
mm->mmap is set to NULL so that nobody will access the freed vma list.

Signed-off-by: Michal Hocko <mhocko@suse.com>
---
 mm/mmap.c | 23 +----------------------
 1 file changed, 1 insertion(+), 22 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index f4e09d390a07..0d6af9d89aa8 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -3129,28 +3129,6 @@ void exit_mmap(struct mm_struct *mm)
 	/* mm's last user has gone, and its about to be pulled down */
 	mmu_notifier_release(mm);
 
-	if (unlikely(mm_is_oom_victim(mm))) {
-		/*
-		 * Manually reap the mm to free as much memory as possible.
-		 * Then, as the oom reaper does, set MMF_OOM_SKIP to disregard
-		 * this mm from further consideration.  Taking mm->mmap_lock for
-		 * write after setting MMF_OOM_SKIP will guarantee that the oom
-		 * reaper will not run on this mm again after mmap_lock is
-		 * dropped.
-		 *
-		 * Nothing can be holding mm->mmap_lock here and the above call
-		 * to mmu_notifier_release(mm) ensures mmu notifier callbacks in
-		 * __oom_reap_task_mm() will not block.
-		 *
-		 * This needs to be done before calling unlock_range(),
-		 * which clears VM_LOCKED, otherwise the oom reaper cannot
-		 * reliably test it.
-		 */
-		(void)__oom_reap_task_mm(mm);
-
-		set_bit(MMF_OOM_SKIP, &mm->flags);
-	}
-
 	mmap_write_lock(mm);
 	if (mm->locked_vm)
 		unlock_range(mm->mmap, ULONG_MAX);
@@ -3180,6 +3158,7 @@ void exit_mmap(struct mm_struct *mm)
 		vma = remove_vma(vma);
 		cond_resched();
 	}
+	mm->mmap = NULL;
 	mmap_write_unlock(mm);
 	vm_unacct_memory(nr_accounted);
 }
-- 
2.30.2

-- 
Michal Hocko
SUSE Labs