On 03/08/2016 02:42 PM, Michal Hocko wrote:
> From: Michal Hocko <mhocko@xxxxxxxx>
>
> should_reclaim_retry will give up retries for higher order allocations
> if none of the eligible zones has any requested or higher order pages
> available even if we pass the watermak check for order-0. This is done
> because there is no guarantee that the reclaimable and currently free
> pages will form the required order.
>
> This can, however, lead to situations were the high-order request (e.g.
> order-2 required for the stack allocation during fork) will trigger
> OOM too early - e.g. after the first reclaim/compaction round. Such a
> system would have to be highly fragmented and there is no guarantee
> further reclaim/compaction attempts would help but at least make sure
> that the compaction was active before we go OOM and keep retrying even
> if should_reclaim_retry tells us to oom if the last compaction round
> was either inactive (deferred, skipped or bailed out early due to
> contention) or it told us to continue.
>
> Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
> ---
> include/linux/compaction.h | 5 +++++
> mm/page_alloc.c | 53 ++++++++++++++++++++++++++++++++--------------
> 2 files changed, 42 insertions(+), 16 deletions(-)
>
> diff --git a/include/linux/compaction.h b/include/linux/compaction.h
> index b167801187e7..49e04326dcb8 100644
> --- a/include/linux/compaction.h
> +++ b/include/linux/compaction.h
> @@ -14,6 +14,11 @@ enum compact_result {
> /* compaction should continue to another pageblock */
> COMPACT_CONTINUE,
> /*
> + * whoever is calling compaction should retry because it was either
> + * not active or it tells us there is more work to be done.
> + */
> + COMPACT_SHOULD_RETRY = COMPACT_CONTINUE,
Hmm, I'm not sure about this. AFAIK compact_zone() doesn't ever return
COMPACT_CONTINUE, and thus try_to_compact_pages() also doesn't. This
overloading of CONTINUE only applies to compaction_suitable(). But the
value that should_compact_retry() is testing comes only from
try_to_compact_pages(). So this is not wrong, but perhaps a bit misleading?
> + /*
> * direct compaction partially compacted a zone and there are suitable
> * pages
> */
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 4acc0aa1aee0..041aeb1dc3b4 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2819,28 +2819,20 @@ static struct page *
> __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order,
> int alloc_flags, const struct alloc_context *ac,
> enum migrate_mode mode, int *contended_compaction,
> - bool *deferred_compaction)
> + enum compact_result *compact_result)
> {
> - enum compact_result compact_result;
> struct page *page;
>
> if (!order)
> return NULL;
>
> current->flags |= PF_MEMALLOC;
> - compact_result = try_to_compact_pages(gfp_mask, order, alloc_flags, ac,
> + *compact_result = try_to_compact_pages(gfp_mask, order, alloc_flags, ac,
> mode, contended_compaction);
> current->flags &= ~PF_MEMALLOC;
>
> - switch (compact_result) {
> - case COMPACT_DEFERRED:
> - *deferred_compaction = true;
> - /* fall-through */
> - case COMPACT_SKIPPED:
> + if (*compact_result <= COMPACT_SKIPPED)
> return NULL;
> - default:
> - break;
> - }
>
> /*
> * At least in one zone compaction wasn't deferred or skipped, so let's
> @@ -2870,15 +2862,41 @@ __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order,
>
> return NULL;
> }
> +
> +static inline bool
> + (unsigned int order, enum compact_result compact_result,
> + int contended_compaction)
> +{
> + /*
> + * !costly allocations are really important and we have to make sure
> + * the compaction wasn't deferred or didn't bail out early due to locks
> + * contention before we go OOM.
> + */
> + if (order && order <= PAGE_ALLOC_COSTLY_ORDER) {
> + if (compact_result <= COMPACT_SHOULD_RETRY)
> + return true;
> + if (contended_compaction > COMPACT_CONTENDED_NONE)
> + return true;
> + }
> +
> + return false;
> +}
> #else
> static inline struct page *
> __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order,
> int alloc_flags, const struct alloc_context *ac,
> enum migrate_mode mode, int *contended_compaction,
> - bool *deferred_compaction)
> + enum compact_result *compact_result)
> {
> return NULL;
> }
> +
> +static inline bool
> +should_compact_retry(unsigned int order, enum compact_result compact_result,
> + int contended_compaction)
> +{
> + return false;
> +}
> #endif /* CONFIG_COMPACTION */
>
> /* Perform direct synchronous page reclaim */
> @@ -3118,7 +3136,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> int alloc_flags;
> unsigned long did_some_progress;
> enum migrate_mode migration_mode = MIGRATE_ASYNC;
> - bool deferred_compaction = false;
> + enum compact_result compact_result;
> int contended_compaction = COMPACT_CONTENDED_NONE;
> int no_progress_loops = 0;
>
> @@ -3227,7 +3245,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> page = __alloc_pages_direct_compact(gfp_mask, order, alloc_flags, ac,
> migration_mode,
> &contended_compaction,
> - &deferred_compaction);
> + &compact_result);
> if (page)
> goto got_pg;
>
> @@ -3240,7 +3258,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> * to heavily disrupt the system, so we fail the allocation
> * instead of entering direct reclaim.
> */
> - if (deferred_compaction)
> + if (compact_result == COMPACT_DEFERRED)
> goto nopage;
>
> /*
> @@ -3294,6 +3312,9 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> did_some_progress > 0, no_progress_loops))
> goto retry;
>
> + if (should_compact_retry(order, compact_result, contended_compaction))
> + goto retry;
> +
> /* Reclaim has failed us, start killing things */
> page = __alloc_pages_may_oom(gfp_mask, order, ac, &did_some_progress);
> if (page)
> @@ -3314,7 +3335,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> page = __alloc_pages_direct_compact(gfp_mask, order, alloc_flags,
> ac, migration_mode,
> &contended_compaction,
> - &deferred_compaction);
> + &compact_result);
> if (page)
> goto got_pg;
> nopage:
>
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