Re: crash during oom reaper (was: Re: [PATCH 4/4] [RFC!] mm: 'struct mm_struct' reference counting debugging)

On Fri, Dec 16, 2016 at 11:11:13AM +0100, Michal Hocko wrote:

On Fri 16-12-16 10:43:52, Vegard Nossum wrote:
[...]

quoted

I don't think it's a bug in the OOM reaper itself, but either of the
following two patches will fix the problem (without my understand how or
why):

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index ec9f11d4f094..37b14b2e2af4 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c

@@ -485,7 +485,7 @@ static bool __oom_reap_task_mm(struct task_struct *tsk,

struct mm_struct *mm)
 	 */
 	mutex_lock(&oom_lock);

-	if (!down_read_trylock(&mm->mmap_sem)) {
+	if (!down_write_trylock(&mm->mmap_sem)) {

__oom_reap_task_mm is basically the same thing as MADV_DONTNEED and that
doesn't require the exlusive mmap_sem. So this looks correct to me.

BTW, shouldn't we filter out all VM_SPECIAL VMAs there? Or VM_PFNMAP at
least.

MADV_DONTNEED doesn't touch VM_PFNMAP, but I don't see anything matching
on __oom_reap_task_mm() side.

Other difference is that you use unmap_page_range() witch doesn't touch
mmu_notifiers. MADV_DONTNEED goes via zap_page_range(), which invalidates
the range. Not sure if it can make any difference here.

-- 
 Kirill A. Shutemov

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