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Date: Fri, 22 Jan 2016 17:20:50 +0200
From: "Kirill A. Shutemov" <kirill@shutemov.name>
Subject: Re: [PATCH 2/3] thp: change deferred_split_count() to return number
 of THP in queue
Message-ID: <20160122152049.GA24420@node.shutemov.name>
References: <20160121012237.GE7119@redhat.com>
 <1453378163-133609-1-git-send-email-kirill.shutemov@linux.intel.com>
 <1453378163-133609-3-git-send-email-kirill.shutemov@linux.intel.com>
 <20160122143127.GI7119@redhat.com>
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To: Andrea Arcangeli <aarcange@redhat.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>, Andrew Morton <akpm@linux-foundation.org>, Hugh Dickins <hughd@google.com>, Dave Hansen <dave.hansen@intel.com>, Mel Gorman <mgorman@suse.de>, Rik van Riel <riel@redhat.com>, Vlastimil Babka <vbabka@suse.cz>, Christoph Lameter <cl@gentwo.org>, Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>, Steve Capper <steve.capper@linaro.org>, "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>, Johannes Weiner <hannes@cmpxchg.org>, Michal Hocko <mhocko@suse.cz>, Jerome Marchand <jmarchan@redhat.com>, Sasha Levin <sasha.levin@oracle.com>, linux-kernel@vger.kernel.org, linux-mm@kvack.org

On Fri, Jan 22, 2016 at 03:31:27PM +0100, Andrea Arcangeli wrote:
> On Thu, Jan 21, 2016 at 03:09:22PM +0300, Kirill A. Shutemov wrote:
> > @@ -3511,7 +3506,7 @@ static unsigned long deferred_split_scan(struct shrinker *shrink,
> >  	list_splice_tail(&list, &pgdata->split_queue);
> >  	spin_unlock_irqrestore(&pgdata->split_queue_lock, flags);
> >  
> > -	return split * HPAGE_PMD_NR / 2;
> > +	return split;
> >  }
> 
> Looking further at how the caller processes this "split" retval, if
> the list has been fully shrunk by the page freeing, between the
> split_count and split_scan, the caller seems to ignore a 0 value
> returned above and it'll keep calling even if sc->nr_to_scan isn't
> decreasing. The caller won't even check sc->nr_to_scan to notice that
> it isn't decreasing anymore, it's write-only as far as the caller is
> concerned.
> 
> It's also weird we can't return the number of freed pages and break
> the loop with just one invocation of the split_scan, but that's a
> slight inefficiency in the caller interface. The caller also seems to
> forget to set total_scan to 0 if SHRINK_STOP was returned but perhaps
> that's on purpose, however for our purpose it'd be better off if it
> did.
> 
> The split_queue.next is going to be hot in the CPU cache anyway, so
> unless we change the caller, it should be worth it to add a list_empty
> check and return SHRINK_STOP if it was empty. Doing it at the start or
> end doesn't make much difference, at the end lockless it'll deal with
> the split failures too if any.
> 
> 	return split ? : list_empty(&pgdat->split_queue) ? SPLIT_STOP : 0;

Ughh. Shrinker interface is confusing.