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Date:	Mon, 28 Jul 2014 16:39:03 -0700 (PDT)
From:	David Rientjes <>
To:	Vlastimil Babka <>
cc:	Andrew Morton <>,, linux-mm:;
Subject: Re: [PATCH v5 01/14] mm, THP: don't hold mmap_sem in khugepaged when
 allocating THP

On Mon, 28 Jul 2014, 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 also deletes an outdated comment that assumed vma must be stable, as it was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 9f1b868a13ac
> ("mm: thp: khugepaged: add policy for finding target node").
> Signed-off-by: Vlastimil Babka <>
> Cc: Minchan Kim <>
> Acked-by: Mel Gorman <>
> Cc: Joonsoo Kim <>
> Cc: Michal Nazarewicz <>
> Cc: Naoya Horiguchi <>
> Cc: Christoph Lameter <>
> Cc: Rik van Riel <>
> Cc: David Rientjes <>

Acked-by: David Rientjes <>
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