>
> From: Michal Hocko <mhocko@xxxxxxxx>
>
> THP requests skip the direct reclaim if the compaction is either
> deferred or contended to reduce stalls which wouldn't help the
> allocation success anyway. These checks are ignoring other potential
> feedback modes which we have available now.
>
> It clearly doesn't make much sense to go and reclaim few pages if the
> previous compaction has failed.
>
> We can also simplify the check by using compaction_withdrawn which
> checks for both COMPACT_CONTENDED and COMPACT_DEFERRED. This check
> is however covering more reasons why the compaction was withdrawn.
> None of them should be a problem for the THP case though.
>
> It is safe to back of if we see COMPACT_SKIPPED because that means
> that compaction_suitable failed and a single round of the reclaim is
> unlikely to make any difference here. We would have to be close to
> the low watermark to reclaim enough and even then there is no guarantee
> that the compaction would make any progress while the direct reclaim
> would have caused the stall.
>
> COMPACT_PARTIAL_SKIPPED is slightly different because that means that we
> have only seen a part of the zone so a retry would make some sense. But
> it would be a compaction retry not a reclaim retry to perform. We are
> not doing that and that might indeed lead to situations where THP fails
> but this should happen only rarely and it would be really hard to
> measure.
>
> Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
> ---
Acked-by: Hillf Danton <hillf.zj@xxxxxxxxxxxxxxx>
> mm/page_alloc.c | 27 ++++++++-------------------
> 1 file changed, 8 insertions(+), 19 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 350d13f3709b..d551fe326c33 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3257,25 +3257,14 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> if (page)
> goto got_pg;
>
> - /* Checks for THP-specific high-order allocations */
> - if (is_thp_gfp_mask(gfp_mask)) {
> - /*
> - * If compaction is deferred for high-order allocations, it is
> - * because sync compaction recently failed. If this is the case
> - * and the caller requested a THP allocation, we do not want
> - * to heavily disrupt the system, so we fail the allocation
> - * instead of entering direct reclaim.
> - */
> - if (compact_result == COMPACT_DEFERRED)
> - goto nopage;
> -
> - /*
> - * Compaction is contended so rather back off than cause
> - * excessive stalls.
> - */
> - if(compact_result == COMPACT_CONTENDED)
> - goto nopage;
> - }
> + /*
> + * Checks for THP-specific high-order allocations and back off
> + * if the the compaction backed off or failed
> + */
Alternatively,
/*
* Check THP allocations and back off
* if the compaction bailed out or failed
*/
> + if (is_thp_gfp_mask(gfp_mask) &&
> + (compaction_withdrawn(compact_result) ||
> + compaction_failed(compact_result)))
> + goto nopage;
>
> /*
> * It can become very expensive to allocate transparent hugepages at
> --
> 2.8.0.rc3
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