linux-kernel - Re: [PATCH v3 1/3] mm/free_pcppages

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Date:   Tue, 27 Feb 2018 09:56:13 +0800
From:   Aaron Lu <aaron.lu@...el.com>
To:     David Rientjes <rientjes@...gle.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Huang Ying <ying.huang@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Kemi Wang <kemi.wang@...el.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Michal Hocko <mhocko@...e.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH v3 1/3] mm/free_pcppages_bulk: update pcp->count inside

On Mon, Feb 26, 2018 at 01:48:14PM -0800, David Rientjes wrote:
> 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@...radead.org>
> > Signed-off-by: Aaron Lu <aaron.lu@...el.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?

When count is still count and not zero after 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.

Agree that it looks a bit weird to do it beforehand and I just want to
avoid adding one more local variable here.

pcp->count is not protected by zone->lock though so even we do it after
dropping the lock, it could still happen that zoneinfo shows a wrong
value of pcp->count while it should be zero(this isn't a problem since
zoneinfo doesn't need to be precise).

Anyway, I'll follow your suggestion here to avoid confusion.
 
> Otherwise, looks good.

Thanks for taking a look at this.