linux-kernel - Re: [PATCH v2 00/20] mm, hugetlb: remove a hugetlb_instantiation

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Date:	Wed, 14 Aug 2013 16:22:25 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>
Cc:	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>,
	David Gibson <david@...son.dropbear.id.au>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Joonsoo Kim <js1304@...il.com>,
	Wanpeng Li <liwanp@...ux.vnet.ibm.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Hillf Danton <dhillf@...il.com>
Subject: Re: [PATCH v2 00/20] mm, hugetlb: remove a
 hugetlb_instantiation_mutex

On Fri,  9 Aug 2013 18:26:18 +0900 Joonsoo Kim <iamjoonsoo.kim@....com> wrote:

> Without a hugetlb_instantiation_mutex, 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 to get a SIGBUS
> signal, although there are enough hugepages.
> 
> 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 problem reported by Davidlohr Bueso [1].
> 
> This patchset consist of 4 parts roughly.
> 
> Part 1. (1-6) Random fix and clean-up. Enhancing error handling.
> 	
> 	These can be merged into mainline separately.
> 
> Part 2. (7-9) Protect region tracking via it's own spinlock, instead of
> 	the hugetlb_instantiation_mutex.
> 	
> 	Breaking dependency on the hugetlb_instantiation_mutex for
> 	tracking a region is also needed by other approaches like as
> 	'table mutexes', so these can be merged into mainline separately.
> 
> Part 3. (10-13) Clean-up.
> 	
> 	IMO, these make code really simple, so these are worth to go into
> 	mainline separately, regardless success of my approach.
> 
> Part 4. (14-20) Remove a hugetlb_instantiation_mutex.
> 	
> 	Almost patches are just for clean-up to error handling path.
> 	In patch 19, retry approach is implemented that if faulted thread
> 	failed to allocate a hugepage, it continue to run a fault handler
> 	until there is no concurrent thread having a hugepage. This causes
> 	threads who want to get a last hugepage to be serialized, so
> 	threads don't get a SIGBUS if enough hugepage exist.
> 	In patch 20, remove a hugetlb_instantiation_mutex.

I grabbed the first six easy ones.  I'm getting a bit cross-eyed from
all the reviewing lately so I'll wait and see if someone else takes an
interest in the other patches, sorry.
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