linux-kernel - Re: [PATCH v2 4/8] mm/isolation: close the two race problems related to pageblock isolation

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Date:	Wed, 13 Aug 2014 17:09:56 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Vlastimil Babka <vbabka@...e.cz>
Cc:	Minchan Kim <minchan@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
	Johannes Weiner <hannes@...xchg.org>,
	Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>,
	Zhang Yanfei <zhangyanfei@...fujitsu.com>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	Tang Chen <tangchen@...fujitsu.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
	Wen Congyang <wency@...fujitsu.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Michal Nazarewicz <mina86@...a86.com>,
	Laura Abbott <lauraa@...eaurora.org>,
	Heesub Shin <heesub.shin@...sung.com>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Ritesh Harjani <ritesh.list@...il.com>,
	t.stanislaws@...sung.com, Gioh Kim <gioh.kim@....com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 4/8] mm/isolation: close the two race problems related
 to pageblock isolation

On Tue, Aug 12, 2014 at 11:45:32AM +0200, Vlastimil Babka wrote:
> On 08/12/2014 07:17 AM, Minchan Kim wrote:
> >On Wed, Aug 06, 2014 at 04:18:33PM +0900, Joonsoo Kim wrote:
> >>
> >>One solution to this problem is checking pageblock migratetype with
> >>holding zone lock in __free_one_page() and I posted it before, but,
> >>it didn't get welcome since it needs the hook in zone lock critical
> >>section on freepath.
> >
> >I didn't review your v1 but IMHO, this patchset is rather complex.
> 
> It is, but the complexity is in the isolation code, and not fast
> paths, so that's justifiable IMHO.
> 
> >Normally, we don't like adding more overhead in fast path but we did
> >several time on hotplug/cma, esp so I don't know a few more thing is
> >really hesitant.
> 
> This actually undoes most of the overhead, so I'm all for it. Better
> than keep doing stuff the same way just because it was done
> previously.
> 
> >In addition, you proved by this patchset how this
> >isolation code looks ugly and fragile for race problem so I vote
> >adding more overhead in fast path if it can make code really simple.
> 
> Well, I recommend you to check out the v1 then :) That wasn't really
> simple, that was even more hooks rechecking migratetypes at various
> places of the fast paths, when merging buddies etc. This is much
> better. The complexity is mostly in the isolation code, and the
> overhead happens only during isolation.

Hmm... Okay.

I agree that this way is so complicated. In fact, the real save is just
one is_migrate_isolate_page() check in free_pcppages_bulk() and
this approach makes isolation process really complicated.

I guess that I could improve v1 patchset. How about waiting my
improved v1 and comparing v1' with v2?

If v1 is implemented cleanly, it may be better than this.
I want to try and compare. :)

Thanks.
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