linux-kernel - Re: [PATCH] mm, page_alloc: do not break __GFP

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Date:   Mon, 28 May 2018 09:49:40 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Michal Hocko <mhocko@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>, stable@...r.kernel.org
Subject: Re: [PATCH] mm, page_alloc: do not break __GFP_THISNODE by zonelist
 reset

On Fri, May 25, 2018 at 03:08:53PM +0200, Vlastimil Babka wrote:
> In __alloc_pages_slowpath() we reset zonelist and preferred_zoneref for
> allocations that can ignore memory policies. The zonelist is obtained from
> current CPU's node. This is a problem for __GFP_THISNODE allocations that want
> to allocate on a different node, e.g. because the allocating thread has been
> migrated to a different CPU.
> 
> This has been observed to break SLAB in our 4.4-based kernel, because there it
> relies on __GFP_THISNODE working as intended. If a slab page is put on wrong
> node's list, then further list manipulations may corrupt the list because
> page_to_nid() is used to determine which node's list_lock should be locked and
> thus we may take a wrong lock and race.
> 
> Current SLAB implementation seems to be immune by luck thanks to commit
> 511e3a058812 ("mm/slab: make cache_grow() handle the page allocated on
> arbitrary node") but there may be others assuming that __GFP_THISNODE works as
> promised.
> 
> We can fix it by simply removing the zonelist reset completely. There is
> actually no reason to reset it, because memory policies and cpusets don't
> affect the zonelist choice in the first place. This was different when commit
> 183f6371aac2 ("mm: ignore mempolicies when using ALLOC_NO_WATERMARK")
> introduced the code, as mempolicies provided their own restricted zonelists.
> 
> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
> Fixes: 183f6371aac2 ("mm: ignore mempolicies when using ALLOC_NO_WATERMARK")

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

Thanks.

-- 
Mel Gorman
SUSE Labs