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@xxxxxxxxx> > Signed-off-by: Mel Gorman <mgorman@xxxxxxx> Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx> -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>