linux-kernel - Re: [PATCH v2 19/20] mm, hugetlb: retry if failed to allocate and there is concurrent user

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Date:	Thu, 5 Sep 2013 11:16:11 +1000
From:	David Gibson <david@...son.dropbear.id.au>
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>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Hugh Dickins <hughd@...gle.com>,
	Davidlohr Bueso <davidlohr.bueso@...com>, 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 19/20] mm, hugetlb: retry if failed to allocate and
 there is concurrent user

On Wed, Sep 04, 2013 at 05:44:30PM +0900, Joonsoo Kim wrote:
> On Fri, Aug 09, 2013 at 06:26:37PM +0900, Joonsoo Kim wrote:
> > If parallel fault occur, we can fail to allocate a hugepage,
> > because many threads dequeue a hugepage to handle a fault of same address.
> > This makes reserved pool shortage just for a little while and this cause
> > faulting thread who can get hugepages to get a SIGBUS signal.
> > 
> > To solve this problem, we already have a nice solution, that is,
> > a hugetlb_instantiation_mutex. This blocks other threads to dive into
> > a fault handler. This solve the problem clearly, but it introduce
> > performance degradation, because it serialize all fault handling.
> > 
> > Now, I try to remove a hugetlb_instantiation_mutex to get rid of
> > performance degradation. For achieving it, at first, we should ensure that
> > no one get a SIGBUS if there are enough hugepages.
> > 
> > For this purpose, if we fail to allocate a new hugepage when there is
> > concurrent user, we return just 0, instead of VM_FAULT_SIGBUS. With this,
> > these threads defer to get a SIGBUS signal until there is no
> > concurrent user, and so, we can ensure that no one get a SIGBUS if there
> > are enough hugepages.
> > 
> > Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@....com>
> > 
> 
> Hello, David.
> May I ask to you to review this one?
> I guess that you already thought about the various race condition,
> so I think that you are the most appropriate reviewer to this patch. :)

Yeah, sorry, I meant to get to it but kept forgetting.  I've sent a
review now.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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