On Thu, Oct 04, 2012 at 01:50:48AM +0200, Andrea Arcangeli wrote:
> In the special "pmd" mode of knuma_scand
> (/sys/kernel/mm/autonuma/knuma_scand/pmd == 1), the pmd may be of numa
> type (_PAGE_PRESENT not set), however the pte might be
> present. Therefore, gup_pmd_range() must return 0 in this case to
> avoid losing a NUMA hinting page fault during gup_fast.
>
So if gup_fast fails, presumably we fall back to taking the mmap_sem and
calling get_user_pages(). This is a heavier operation and I wonder if the
cost is justified. i.e. Is the performance loss from using get_user_pages()
offset by improved NUMA placement? I ask because we always incur the cost of
taking mmap_sem but only sometimes get it back from improved NUMA placement.
How bad would it be if gup_fast lost some of the NUMA hinting information?
> Note: gup_fast will skip over non present ptes (like numa types), so
> no explicit check is needed for the pte_numa case. gup_fast will also
> skip over THP when the trans huge pmd is non present. So, the pmd_numa
> case will also be correctly skipped with no additional code changes
> required.
>
> Acked-by: Rik van Riel <riel@xxxxxxxxxx>
> Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> ---
> arch/x86/mm/gup.c | 13 ++++++++++++-
> 1 files changed, 12 insertions(+), 1 deletions(-)
>
> diff --git a/arch/x86/mm/gup.c b/arch/x86/mm/gup.c
> index 6dc9921..cad7d97 100644
> --- a/arch/x86/mm/gup.c
> +++ b/arch/x86/mm/gup.c
> @@ -169,8 +169,19 @@ static int gup_pmd_range(pud_t pud, unsigned long addr, unsigned long end,
> * can't because it has irq disabled and
> * wait_split_huge_page() would never return as the
> * tlb flush IPI wouldn't run.
> + *
> + * The pmd_numa() check is needed because the code
> + * doesn't check the _PAGE_PRESENT bit of the pmd if
> + * the gup_pte_range() path is taken. NOTE: not all
> + * gup_fast users will will access the page contents
> + * using the CPU through the NUMA memory channels like
> + * KVM does. So we're forced to trigger NUMA hinting
> + * page faults unconditionally for all gup_fast users
> + * even though NUMA hinting page faults aren't useful
> + * to I/O drivers that will access the page with DMA
> + * and not with the CPU.
> */
> - if (pmd_none(pmd) || pmd_trans_splitting(pmd))
> + if (pmd_none(pmd) || pmd_trans_splitting(pmd) || pmd_numa(pmd))
> return 0;
> if (unlikely(pmd_large(pmd))) {
> if (!gup_huge_pmd(pmd, addr, next, write, pages, nr))
>
--
Mel Gorman
SUSE Labs
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