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 also deletes an outdated comment that assumed vma must be stable, as it was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 <vba...@suse.cz>
> Cc: Minchan Kim <minc...@kernel.org>
> Acked-by: Mel Gorman <mgor...@suse.de>
> Cc: Joonsoo Kim <iamjoonsoo....@lge.com>
> Cc: Michal Nazarewicz <min...@mina86.com>
> Cc: Naoya Horiguchi <n-horigu...@ah.jp.nec.com>
> Cc: Christoph Lameter <c...@linux.com>
> Cc: Rik van Riel <r...@redhat.com>
> Cc: David Rientjes <rient...@google.com>
Acked-by: David Rientjes <rient...@google.com>
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