linux-kernel - Re: [PATCH 1/2] hugetlb: properly account rss

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Date:	Wed, 3 Jul 2013 14:41:19 +0100
From:	Steve Capper <steve.capper@...aro.org>
To:	Joern Engel <joern@...fs.org>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 1/2] hugetlb: properly account rss

On Tue, Jun 18, 2013 at 02:47:04PM -0400, Joern Engel wrote:
> When moving a program from mmap'ing small pages to mmap'ing huge pages,
> a remarkable drop in rss ensues.  For some reason hugepages were never
> accounted for in rss, which in my book is a clear bug.  Sadly this bug
> has been present in hugetlbfs since it was merged back in 2002.  There
> is every chance existing programs depend on hugepages not being counted
> as rss.
> 
> I think the correct solution is to fix the bug and wait for someone to
> complain.  It is just as likely that noone cares - as evidenced by the
> fact that noone seems to have noticed for ten years.
> 
> Signed-off-by: Joern Engel <joern@...fs.org>
> ---
>  mm/hugetlb.c |    4 ++++
>  1 file changed, 4 insertions(+)
> 

Hi,
This patch has caused a few warnings for me today when it was integrated into
linux-next. The libhugetlbfs test suite gave me:
[ 94.320661] BUG: Bad rss-counter state mm:ffff880119461040 idx:1 val:-512
[ 94.330346] BUG: Bad rss-counter state mm:ffff880119460680 idx:1 val:-2560
[ 94.341746] BUG: Bad rss-counter state mm:ffff880119460d00 idx:1 val:-512
[ 94.347518] BUG: Bad rss-counter state mm:ffff880119460d00 idx:1 val:-512
[ 94.415203] BUG: Bad rss-counter state mm:ffff8801194f9040 idx:1 val:-1024

[ ...]

I think I've found the cause; MAP_SHARED mappings.
alloc_huge_page and __unmap_hugepage_range are called for shared pages. Also,
__unmap_hugepage_range is called more times than alloc_huge_page (which makes
sense as multiple views of a shared mapping are unmapped) leading to negative
counter values.

Excluding VM_SHARED VMAs from the counter increment/decrement stopped the
warnings for me. Although this may not be the best way to address the issue.

Cheers,
-- 
Steve
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