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

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Date:   Fri, 7 Oct 2016 23:30:25 +0900
From:   Minchan Kim <minchan@...nel.org>
To:     Vlastimil Babka <vbabka@...e.cz>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        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 02:44:15PM +0200, Vlastimil Babka wrote:
> On 10/07/2016 07:45 AM, Minchan Kim wrote:
> >There is race between page freeing and unreserved highatomic.
> >
> > CPU 0				    CPU 1
> >
> >    free_hot_cold_page
> >      mt = get_pfnblock_migratetype
> 
> so here mt == MIGRATE_HIGHATOMIC?

Yes.

> 
> >      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.
> 
> Hmm I see.
> 
> >So, this patch verifies whether the pageblock is highatomic or not
> >and decrease the count only if the pageblock is highatomic.
> 
> Yeah I guess that's the easiest solution.
> 
> >Signed-off-by: Minchan Kim <minchan@...nel.org>
> 
> Acked-by: Vlastimil Babka <vbabka@...e.cz>

Thanks, Vlastimil.