Subject: Re: BUG: non-zero nr_pmds on freeing mm: 1

From: "Kirill A. Shutemov" <[email-masked]>
Date: Mon, 9 Feb 2015 18:42:48 +0200

On Sat, Feb 07, 2015 at 08:33:02AM +0100, Sedat Dilek wrote:
> On Sat, Feb 7, 2015 at 6:12 AM, Pat Erley <[email-masked]> wrote:
> > I'm seeing the message in $subject on my Xen DOM0 on next-20150204 on
> > x86_64.  I haven't had time to bisect it, but have seen some discussion on
> > similar topics here recently.  I can trigger this pretty reliably by
> > watching Netflix.  At some point (minutes to hours) into it, the netflix
> > video goes black (audio keeps going, so it still thinks it's working) and
> > the error appears in dmesg.  Refreshing the page gets the video going again,
> > and it will continue playing for some indeterminate amount of time.
> >
> > Kirill, I've CC'd you as looking in the logs, you've patched a false
> > positive trigger of this very recently(patch in kernel I'm running).  Am I
> > actually hitting a problem, or is this another false positive case? Any
> > additional details that might help?
> >
> > Dmesg from system attached.
> 
> [ CC some mm folks ]
> 
> I have seen this, too.
> 
> root# grep "BUG: non-zero nr_pmds on freeing mm:" /var/log/kern.log | wc -l
> 21
> 
> Checking my logs: On next-20150203 and next-20150204.
> 
> I am here not in a VM environment and cannot say what causes these messages.

Sorry, my fault.

The patch below should fix that.

>>From 11bce596e653302e41f819435912f01ca8cbc27e Mon Sep 17 00:00:00 2001
From: "Kirill A. Shutemov" <[email-masked]>
Date: Mon, 9 Feb 2015 18:34:56 +0200
Subject: [PATCH] mm: fix race on pmd accounting

Do not account the pmd table to the process if other thread allocated it
under us.

Signed-off-by: Kirill A. Shutemov <[email-masked]>
Reported-by: Sedat Dilek <[email-masked]>
---
 mm/memory.c | 15 ++++++++-------
 1 file changed, 8 insertions(+), 7 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 8ae52c918415..802adda2b0b6 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3350,17 +3350,18 @@ int __pmd_alloc(struct mm_struct *mm, pud_t *pud, unsigned long address)
 	smp_wmb(); /* See comment in __pte_alloc */
 
 	spin_lock(&mm->page_table_lock);
-	mm_inc_nr_pmds(mm);
 #ifndef __ARCH_HAS_4LEVEL_HACK
-	if (pud_present(*pud))		/* Another has populated it */
-		pmd_free(mm, new);
-	else
+	if (!pud_present(*pud)) {
+		mm_inc_nr_pmds(mm);
 		pud_populate(mm, pud, new);
-#else
-	if (pgd_present(*pud))		/* Another has populated it */
+	} else	/* Another has populated it */
 		pmd_free(mm, new);
-	else
+#else
+	if (!pgd_present(*pud)) {
+		mm_inc_nr_pmds(mm);
 		pgd_populate(mm, pud, new);
+	} else /* Another has populated it */
+		pmd_free(mm, new);
 #endif /* __ARCH_HAS_4LEVEL_HACK */
 	spin_unlock(&mm->page_table_lock);
 	return 0;
-- 
 Kirill A. Shutemov

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