On Wed, 18 Nov 2015, Michal Hocko wrote:
> From: Michal Hocko <mhocko@xxxxxxxx>
>
> __alloc_pages_slowpath retries costly allocations until at least
> order worth of pages were reclaimed or the watermark check for at least
> one zone would succeed after all reclaiming all pages if the reclaim
> hasn't made any progress.
>
> The first condition was added by a41f24ea9fd6 ("page allocator: smarter
> retry of costly-order allocations) and it assumed that lumpy reclaim
> could have created a page of the sufficient order. Lumpy reclaim,
> has been removed quite some time ago so the assumption doesn't hold
> anymore. It would be more appropriate to check the compaction progress
> instead but this patch simply removes the check and relies solely
> on the watermark check.
>
> To prevent from too many retries the stall_backoff is not reseted after
> a reclaim which made progress because we cannot assume it helped high
> order situation.
>
> Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
> ---
> mm/page_alloc.c | 20 ++++++++------------
> 1 file changed, 8 insertions(+), 12 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index e6271bc19e6a..999c8cdbe7b5 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3006,7 +3006,6 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> bool can_direct_reclaim = gfp_mask & __GFP_DIRECT_RECLAIM;
> struct page *page = NULL;
> int alloc_flags;
> - unsigned long pages_reclaimed = 0;
> unsigned long did_some_progress;
> enum migrate_mode migration_mode = MIGRATE_ASYNC;
> bool deferred_compaction = false;
> @@ -3167,24 +3166,21 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
>
> /*
> * Do not retry high order allocations unless they are __GFP_REPEAT
> - * and even then do not retry endlessly unless explicitly told so
> + * unless explicitly told so.
> */
> - pages_reclaimed += did_some_progress;
> - if (order > PAGE_ALLOC_COSTLY_ORDER) {
> - if (!(gfp_mask & __GFP_NOFAIL) &&
> - (!(gfp_mask & __GFP_REPEAT) || pages_reclaimed >= (1<<order)))
> - goto noretry;
> -
> - if (did_some_progress)
> - goto retry;
> - }
> + if (order > PAGE_ALLOC_COSTLY_ORDER &&
> + !(gfp_mask & (__GFP_REPEAT|__GFP_NOFAIL)))
> + goto noretry;
Who is allocating order > PAGE_ALLOC_COSTLY_ORDER with __GFP_REPEAT and
would be affected by this change?
>
> /*
> * Be optimistic and consider all pages on reclaimable LRUs as usable
> * but make sure we converge to OOM if we cannot make any progress after
> * multiple consecutive failed attempts.
> + * Costly __GFP_REPEAT allocations might have made a progress but this
> + * doesn't mean their order will become available due to high fragmentation
> + * so do not reset the backoff for them
> */
> - if (did_some_progress)
> + if (did_some_progress && order <= PAGE_ALLOC_COSTLY_ORDER)
> stall_backoff = 0;
> else
> stall_backoff = min(stall_backoff+1, MAX_STALL_BACKOFF);
This makes sense if there are high-order users of __GFP_REPEAT since
only using a number of pages reclaimed by itself isn't helpful.
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