linux-kernel - Re: [PATCH] mm, compaction: make fast_isolate

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Date:   Wed, 17 Feb 2021 18:50:16 +0000
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org,
        Andrea Arcangeli <aarcange@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        Michal Hocko <mhocko@...nel.org>,
        Mike Rapoport <rppt@...nel.org>, stable@...r.kernel.org
Subject: Re: [PATCH] mm, compaction: make fast_isolate_freepages() stay
 within zone

On Wed, Feb 17, 2021 at 06:33:00PM +0100, Vlastimil Babka wrote:
> Compaction always operates on pages from a single given zone when isolating
> both pages to migrate and freepages. Pageblock boundaries are intersected with
> zone boundaries to be safe in case zone starts or ends in the middle of
> pageblock. The use of pageblock_pfn_to_page() protects against non-contiguous
> pageblocks.
> 
> The functions fast_isolate_freepages() and fast_isolate_around() don't
> currently protect the fast freepage isolation thoroughly enough against these
> corner cases, and can result in freepage isolation operate outside of zone
> boundaries:
> 
> - in fast_isolate_freepages() if we get a pfn from the first pageblock of a
>   zone that starts in the middle of that pageblock, 'highest' can be a pfn
>   outside of the zone. If we fail to isolate anything in this function, we
>   may then call fast_isolate_around() on a pfn outside of the zone and there
>   effectively do a set_pageblock_skip(page_to_pfn(highest)) which may currently
>   hit a VM_BUG_ON() in some configurations
> - fast_isolate_around() checks only the zone end boundary and not beginning,
>   nor that the pageblock is contiguous (with pageblock_pfn_to_page()) so it's
>   possible that we end up calling isolate_freepages_block() on a range of pfn's
>   from two different zones and end up e.g. isolating freepages under the wrong
>   zone's lock.
> 
> This patch should fix the above issues.
> 
> Fixes: 5a811889de10 ("mm, compaction: use free lists to quickly locate a migration target")

Acked-by: Mel Gorman <mgorman@...hsingularity.net>

-- 
Mel Gorman
SUSE Labs