linux-kernel - Re: [PATCH V4 01/15] mm, THP: don't hold mmap

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Date:	Fri, 25 Jul 2014 13:18:15 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Vlastimil Babka <vbabka@...e.cz>
Cc:	linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
	David Rientjes <rientjes@...gle.com>,
	linux-kernel@...r.kernel.org, Minchan Kim <minchan@...nel.org>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Michal Nazarewicz <mina86@...a86.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Christoph Lameter <cl@...ux.com>,
	Rik van Riel <riel@...hat.com>,
	Zhang Yanfei <zhangyanfei@...fujitsu.com>
Subject: Re: [PATCH V4 01/15] mm, THP: don't hold mmap_sem in khugepaged when
 allocating THP

On Wed, Jul 16, 2014 at 03:48:09PM +0200, Vlastimil Babka wrote:
> When allocating huge page for collapsing, khugepaged currently holds mmap_sem
> for reading on the mm where collapsing occurs. Afterwards the read lock is
> dropped before write lock is taken on the same mmap_sem.
> 
> Holding mmap_sem during whole huge page allocation is therefore useless, the
> vma needs to be rechecked after taking the write lock anyway. Furthemore, huge
> page allocation might involve a rather long sync compaction, and thus block
> any mmap_sem writers and i.e. affect workloads that perform frequent m(un)map
> or mprotect oterations.
> 
> This patch simply releases the read lock before allocating a huge page. It
> also deletes an outdated comment that assumed vma must be stable, as it was
> using alloc_hugepage_vma(). This is no longer true since commit 9f1b868a13
> ("mm: thp: khugepaged: add policy for finding target node").
> 
> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>

Acked-by: Mel Gorman <mgorman@...e.de>

-- 
Mel Gorman
SUSE Labs
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