Re: [PATCH] mm: Check if PTE is already allocated during page fault

[Johannes Weiner <hannes@cmpxchg.org>]

On Fri, Apr 15, 2011 at 11:12:48AM +0100, Mel Gorman wrote:
> With transparent hugepage support, handle_mm_fault() has to be careful
> that a normal PMD has been established before handling a PTE fault. To
> achieve this, it used __pte_alloc() directly instead of pte_alloc_map
> as pte_alloc_map is unsafe to run against a huge PMD. pte_offset_map()
> is called once it is known the PMD is safe.
> 
> pte_alloc_map() is smart enough to check if a PTE is already present
> before calling __pte_alloc but this check was lost. As a consequence,
> PTEs may be allocated unnecessarily and the page table lock taken.
> Thi useless PTE does get cleaned up but it's a performance hit which
> is visible in page_test from aim9.
> 
> This patch simply re-adds the check normally done by pte_alloc_map to
> check if the PTE needs to be allocated before taking the page table
> lock. The effect is noticable in page_test from aim9.
> 
> AIM9
>                 2.6.38-vanilla 2.6.38-checkptenone
> creat-clo      446.10 ( 0.00%)   424.47 (-5.10%)
> page_test       38.10 ( 0.00%)    42.04 ( 9.37%)
> brk_test        52.45 ( 0.00%)    51.57 (-1.71%)
> exec_test      382.00 ( 0.00%)   456.90 (16.39%)
> fork_test       60.11 ( 0.00%)    67.79 (11.34%)
> MMTests Statistics: duration
> Total Elapsed Time (seconds)                611.90    612.22
> 
> (While this affects 2.6.38, it is a performance rather than a
> functional bug and normally outside the rules -stable. While the big
> performance differences are to a microbench, the difference in fork
> and exec performance may be significant enough that -stable wants to
> consider the patch)
> 
> Reported-by: Raz Ben Yehuda <raziebe@gmail.com>
> Signed-off-by: Mel Gorman <mgorman@suse.de>

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

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