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Date: Mon, 26 Feb 2018 13:48:14 -0800 (PST)
From: David Rientjes <rientjes@google.com>
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To: Aaron Lu <aaron.lu@intel.com>
cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Huang Ying <ying.huang@intel.com>, Dave Hansen <dave.hansen@intel.com>,
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        Andi Kleen <ak@linux.intel.com>, Michal Hocko <mhocko@suse.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Mel Gorman <mgorman@techsingularity.net>,
        Matthew Wilcox <willy@infradead.org>
Subject: Re: [PATCH v3 1/3] mm/free_pcppages_bulk: update pcp->count inside
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On Mon, 26 Feb 2018, Aaron Lu wrote:

> Matthew Wilcox found that all callers of free_pcppages_bulk() currently
> update pcp->count immediately after so it's natural to do it inside
> free_pcppages_bulk().
> 
> No functionality or performance change is expected from this patch.
> 
> Suggested-by: Matthew Wilcox <willy@infradead.org>
> Signed-off-by: Aaron Lu <aaron.lu@intel.com>
> ---
>  mm/page_alloc.c | 10 +++-------
>  1 file changed, 3 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index cb416723538f..3154859cccd6 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1117,6 +1117,7 @@ static void free_pcppages_bulk(struct zone *zone, int count,
>  	int batch_free = 0;
>  	bool isolated_pageblocks;
>  
> +	pcp->count -= count;
>  	spin_lock(&zone->lock);
>  	isolated_pageblocks = has_isolate_pageblock(zone);
>  

Why modify pcp->count before the pages have actually been freed?

I doubt that it matters too much, but at least /proc/zoneinfo uses 
zone->lock.  I think it should be done after the lock is dropped.

Otherwise, looks good.