linux-kernel - Re: [PATCH] mm/compaction: count pages and stop correctly during page isolation.

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Date:   Fri, 30 Oct 2020 10:43:08 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Zi Yan <ziy@...dia.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        Rik van Riel <riel@...riel.com>,
        linux-kernel@...r.kernel.org, Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH] mm/compaction: count pages and stop correctly during
 page isolation.

[Cc Vlastimil]

On Thu 29-10-20 16:04:35, Zi Yan wrote:
> From: Zi Yan <ziy@...dia.com>
> 
> In isolate_migratepages_block, when cc->alloc_contig is true, we are
> able to isolate compound pages, nr_migratepages and nr_isolated did not
> count compound pages correctly, causing us to isolate more pages than we
> thought. Use thp_nr_pages to count pages. Otherwise, we might be trapped
> in too_many_isolated while loop, since the actual isolated pages can go
> up to COMPACT_CLUSTER_MAX*512=16384, where COMPACT_CLUSTER_MAX is 32,
> since we stop isolation after cc->nr_migratepages reaches to
> COMPACT_CLUSTER_MAX.
> 
> In addition, after we fix the issue above, cc->nr_migratepages could
> never be equal to COMPACT_CLUSTER_MAX if compound pages are isolated,
> thus page isolation could not stop as we intended. Change the isolation
> stop condition to >=.
> 
> Signed-off-by: Zi Yan <ziy@...dia.com>
> ---
>  mm/compaction.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/compaction.c b/mm/compaction.c
> index ee1f8439369e..0683a4999581 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -1012,8 +1012,8 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>  
>  isolate_success:
>  		list_add(&page->lru, &cc->migratepages);
> -		cc->nr_migratepages++;
> -		nr_isolated++;
> +		cc->nr_migratepages += thp_nr_pages(page);
> +		nr_isolated += thp_nr_pages(page);

Does thp_nr_pages work for __PageMovable pages?

-- 
Michal Hocko
SUSE Labs