[RFC 3/3] exit: Check for MMF_OOM_SKIP in exit_mmap

[Nico Pache <npache@redhat.com>]

The MMF_OOM_SKIP bit is used to indicate weather a mm_struct can not be
invalided or has already been invalided. exit_mmap currently calls
__oom_reap_task_mm unconditionally despite the fact that the oom reaper
may have already called this.

Add a check for the MMF_OOM_SKIP bit being set in exit_mmap to avoid
unnessary calls to the invalidate code.

A slight race can occur on the MMF_OOM_SKIP bit that will still allow
this to run twice. My testing has shown an ~66% decrease in double calls
to _oom_reap_task_mm.

Fixes: 27ae357fa82b ("mm, oom: fix concurrent munlock and oom reaper unmap, v3")
Cc: David Rientjes <rientjes@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Nico Pache <npache@redhat.com>
---
 mm/mmap.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index a2968669fd4e..b867f408dacd 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -3113,7 +3113,8 @@ 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))) {
+	if (unlikely(mm_is_oom_victim(mm)) &&
+			!test_bit(MMF_OOM_SKIP, &mm->flags)) {
 		/*
 		 * Manually reap the mm to free as much memory as possible.
 		 * Then, as the oom reaper does, set MMF_OOM_SKIP to disregard
-- 
2.35.1