linux-kernel - Re: [PATCH v2 03/20] mm, hugetlb: fix subpool accounting handling

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Date:	Mon, 26 Aug 2013 18:31:35 +0530
From:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
	Michal Hocko <mhocko@...e.cz>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Hugh Dickins <hughd@...gle.com>,
	Davidlohr Bueso <davidlohr.bueso@...com>,
	David Gibson <david@...son.dropbear.id.au>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Wanpeng Li <liwanp@...ux.vnet.ibm.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Hillf Danton <dhillf@...il.com>
Subject: Re: [PATCH v2 03/20] mm, hugetlb: fix subpool accounting handling

Joonsoo Kim <iamjoonsoo.kim@....com> writes:

> On Thu, Aug 22, 2013 at 12:38:12PM +0530, Aneesh Kumar K.V wrote:
>> Joonsoo Kim <iamjoonsoo.kim@....com> writes:
>> 
>> > Hello, Aneesh.
>> >
>> > First of all, thank you for review!
>> >
>> > On Wed, Aug 21, 2013 at 02:58:20PM +0530, Aneesh Kumar K.V wrote:
>> >> Joonsoo Kim <iamjoonsoo.kim@....com> writes:
>> >> 
>> >> > If we alloc hugepage with avoid_reserve, we don't dequeue reserved one.
>> >> > So, we should check subpool counter when avoid_reserve.
>> >> > This patch implement it.
>> >> 
>> >> Can you explain this better ? ie, if we don't have a reservation in the
>> >> area chg != 0. So why look at avoid_reserve. 
>> >
>> > We don't consider avoid_reserve when chg != 0.
>> > Look at following code.
>> >
>> > +       if (chg || avoid_reserve)
>> > +               if (hugepage_subpool_get_pages(spool, 1))
>> >
>> > It means that if chg != 0, we skip to check avoid_reserve.
>> 
>> when whould be avoid_reserve == 1 and chg == 0 ?
>
> In this case, we should do hugepage_subpool_get_pages(), since we don't
> get a reserved page due to avoid_reserve.

As per off-list discussion we had around this, please add additional
information in commit message explaining when we have
avoid_reserve == 1 and chg == 0

Something like the below copied from call site.

	 /* If the process that created a MAP_PRIVATE mapping is about to
	  * perform a COW due to a shared page count, attempt to satisfy
	  * the allocation without using the existing reserves
          */

Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@...ux.vnet.ibm.com>

-aneesh

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