linux-kernel - Re: [PATCH 2/4] mm: compaction: Only force pageblock scan completion when skip hints are obeyed

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Date:   Thu, 25 May 2023 12:01:18 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        Michal Hocko <mhocko@...nel.org>,
        Pedro Falcato <pedro.falcato@...il.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Chuyi Zhou <zhouchuyi@...edance.com>,
        Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/4] mm: compaction: Only force pageblock scan completion
 when skip hints are obeyed

On 5/15/23 13:33, Mel Gorman wrote:
> fast_find_migrateblock relies on skip hints to avoid rescanning a recently
> selected pageblock but compact_zone() only forces the pageblock scan
> completion to set the skip hint if in direct compaction.  While this
> prevents direct compaction repeatedly scanning a subset of blocks due
> to fast_find_migrateblock(), it does not prevent proactive compaction,
> node compaction and kcompactd encountering the same problem described
> in commit cfccd2e63e7e ("mm, compaction: finish pageblocks on complete
> migration failure").
> 
> Force the scan completion of a pageblock to set the skip hint if skip
> hints are obeyed to prevent fast_find_migrateblock() repeatedly selecting
> a subset of pageblocks.
> 
> Suggested-by: Vlastimil Babka <vbabka@...e.cz>
> Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>

Acked-by: Vlastimil Babka <vbabka@...e.cz>

> ---
>  mm/compaction.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 81791c124bb8..accc6568091a 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -2456,7 +2456,8 @@ compact_zone(struct compact_control *cc, struct capture_control *capc)
>  			}
>  			/*
>  			 * If an ASYNC or SYNC_LIGHT fails to migrate a page
> -			 * within the current order-aligned block, scan the
> +			 * within the current order-aligned block and
> +			 * fast_find_migrateblock may be used then scan the
>  			 * remainder of the pageblock. This will mark the
>  			 * pageblock "skip" to avoid rescanning in the near
>  			 * future. This will isolate more pages than necessary
> @@ -2465,7 +2466,7 @@ compact_zone(struct compact_control *cc, struct capture_control *capc)
>  			 * recently partially scanned.
>  			 */
>  			if (!pageblock_aligned(cc->migrate_pfn) &&
> -			    cc->direct_compaction && !cc->finish_pageblock &&
> +			    !cc->ignore_skip_hint && !cc->finish_pageblock &&
>  			    (cc->mode < MIGRATE_SYNC)) {
>  				cc->finish_pageblock = true;
>