linux-kernel - Re: [patch 5/5]thp: split huge page if head page is isolated

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Message-ID: <20111028073026.GB6268@barrios-laptop.redhat.com>
Date:	Fri, 28 Oct 2011 16:30:26 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	Shaohua Li <shaohua.li@...el.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"aarcange@...hat.com" <aarcange@...hat.com>,
	Hugh Dickins <hughd@...gle.com>,
	Rik van Riel <riel@...hat.com>, mel <mel@....ul.ie>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	linux-mm <linux-mm@...ck.org>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [patch 5/5]thp: split huge page if head page is isolated

On Fri, Oct 28, 2011 at 01:11:55PM +0800, Shaohua Li wrote:
> On Fri, 2011-10-28 at 07:34 +0800, Minchan Kim wrote:
> > On Tue, Oct 25, 2011 at 10:59:40AM +0800, Shaohua Li wrote:
> > > With current logic, if page reclaim finds a huge page, it will just reclaim
> > > the head page and leave tail pages reclaimed later. Let's take an example,
> > > lru list has page A and B, page A is huge page:
> > > 1. page A is isolated
> > > 2. page B is isolated
> > > 3. shrink_page_list() adds page A to swap page cache. so page A is split.
> > > page A+1, page A+2, ... are added to lru list.
> > > 4. shrink_page_list() adds page B to swap page cache.
> > > 5. page A and B is written out and reclaimed.
> > > 6. page A+1, A+2 ... is isolated and reclaimed later.
> > > So the reclaim order is A, B, ...(maybe other pages), A+1, A+2 ...
> > 
> > I don't see your code yet but have a question.
> > You mitigate this problem by 4/5 which could add subpages into lru tail
> > so subpages would reclaim next interation of reclaim.
> > 
> > What do we need 5/5?
> > Do I miss something?
> Both patches are required. without this patch, current page reclaim will
> only reclaim the first page of a huge page, because the hugepage isn't
> split yet. The hugepage is split when the first page is being written to
> swap, which is too later and page reclaim might already isolated a lot
> of pages.

When split happens, subpages would be located in tail of LRU by your 4/5.
(Assume tail of LRU is old age).
In addtion, isolation happens 32 page chunk so the subpages would be isolated
and reclaimed in next iteration. I think 32 pages are not too many.

What do you think about it?
-- 
Kind regards,
Minchan Kim
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