mm: check if PTE is already allocated during page fault

mm: check if PTE is already allocated during page fault

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>
Reviewed-by: Rik van Riel <riel@redhat.com>
Reviewed-by: Andrea Arcangeli <aarcange@redhat.com>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: <stable@kernel.org>		[2.6.38.x]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>