linux-kernel - Re: [PATCH 2/4] mm: prevent double decrease of nr_reserved

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Message-ID: <20161012053611.GB22174@techsingularity.net>
Date:   Wed, 12 Oct 2016 06:36:11 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Sangseok Lee <sangseok.lee@....com>
Subject: Re: [PATCH 2/4] mm: prevent double decrease of nr_reserved_highatomic

On Fri, Oct 07, 2016 at 02:45:34PM +0900, Minchan Kim wrote:
> There is race between page freeing and unreserved highatomic.
> 
>  CPU 0				    CPU 1
> 
>     free_hot_cold_page
>       mt = get_pfnblock_migratetype
>       set_pcppage_migratetype(page, mt)
>     				    unreserve_highatomic_pageblock
>     				    spin_lock_irqsave(&zone->lock)
>     				    move_freepages_block
>     				    set_pageblock_migratetype(page)
>     				    spin_unlock_irqrestore(&zone->lock)
>       free_pcppages_bulk
>         __free_one_page(mt) <- mt is stale
> 
> By above race, a page on CPU 0 could go non-highorderatomic free list
> since the pageblock's type is changed. By that, unreserve logic of
> highorderatomic can decrease reserved count on a same pageblock
> several times and then it will make mismatch between
> nr_reserved_highatomic and the number of reserved pageblock.
> 
> So, this patch verifies whether the pageblock is highatomic or not
> and decrease the count only if the pageblock is highatomic.
> 
> Signed-off-by: Minchan Kim <minchan@...nel.org>

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

-- 
Mel Gorman
SUSE Labs