linux-kernel - Re: [PATCH 2/8] mm/compaction: correct last_migrated_pfn update in compact

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Date:   Tue, 1 Aug 2023 10:09:18 +0800
From:   Baolin Wang <baolin.wang@...ux.alibaba.com>
To:     Kemeng Shi <shikemeng@...weicloud.com>, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        mgorman@...hsingularity.net, willy@...radead.org, david@...hat.com
Subject: Re: [PATCH 2/8] mm/compaction: correct last_migrated_pfn update in
 compact_zone



On 7/29/2023 1:10 AM, Kemeng Shi wrote:
> We record start pfn of last isolated page block with last_migrated_pfn. And
> then:
> 1. We check if we mark the page block skip for exclusive access in
> isolate_migratepages_block by test if next migrate pfn is still in last
> isolated page block. If so, we will set finish_pageblock to do the rescan.
> 2. We check if a full cc->order block is scanned by test if last scan range
> passes the cc->order block boundary. If so, we flush the pages were freed.
> 
> We treat cc->migrate_pfn before isolate_migratepages as the start pfn of
> last isolated page range. However, we always align migrate_pfn to page block
> or move to another page block in fast_find_migrateblock or in linearly scan
> forward in isolate_migratepages before do page isolation in
> isolate_migratepages_block.

Right. But can you describe the impact in detail if the 
last_migrated_pfn is not set correctly? For example, this will result in 
rescan not being set correctly to miss a pageblock's rescanning.

Otherwise looks good to me.

> Update last_migrated_pfn with pageblock_start_pfn(cc->migrate_pfn - 1)
> after scan to correctly set start pfn of last isolated page range. > Signed-off-by: Kemeng Shi <shikemeng@...weicloud.com>
> ---
>   mm/compaction.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/compaction.c b/mm/compaction.c
> index ce7841363b12..fb250c6b2b6e 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -2482,7 +2482,8 @@ compact_zone(struct compact_control *cc, struct capture_control *capc)
>   			goto check_drain;
>   		case ISOLATE_SUCCESS:
>   			update_cached = false;
> -			last_migrated_pfn = iteration_start_pfn;
> +			last_migrated_pfn = max(cc->zone->zone_start_pfn,
> +				pageblock_start_pfn(cc->migrate_pfn - 1));
>   		}
>   
>   		err = migrate_pages(&cc->migratepages, compaction_alloc,