linux-kernel - Re: [patch 4/5]thp: correct order in lru list for split huge page

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Message-ID: <20111027231928.GB29407@barrios-laptop.redhat.com>
Date:	Fri, 28 Oct 2011 08:19:28 +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,
	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 4/5]thp: correct order in lru list for split huge page

On Tue, Oct 25, 2011 at 10:59:37AM +0800, Shaohua Li wrote:
> If a huge page is split, all the subpages should live in lru list adjacently
> because they should be taken as a whole.
> In page split, with current code:
> a. if huge page is in lru list, the order is: page, page+HPAGE_PMD_NR-1,
> page + HPAGE_PMD_NR-2, ..., page + 1(in lru page reclaim order)
> b. otherwise, the order is: page, ..other pages.., page + 1, page + 2, ...(in
> lru page reclaim order). page + 1 ... page + HPAGE_PMD_NR - 1 are in the lru
> reclaim tail.
> 
> In case a, the order is wrong. In case b, page is isolated (to be reclaimed),
> but other tail pages will not soon.
> 
> With below patch:
> in case a, the order is: page, page + 1, ... page + HPAGE_PMD_NR-1(in lru page
> reclaim order).
> in case b, the order is: page + 1, ... page + HPAGE_PMD_NR-1 (in lru page reclaim
> order). The tail pages are in the lru reclaim head.
> 
> Signed-off-by: Shaohua Li <shaohua.li@...el.com>

In case of a, it doesn't matter ordering of subpages.
As a huge page, age of sub pages are same.

In case of b, what a page is located in tail and other subpages are located in head
isn't critical problem.

Having said that, it's more consistent and simple patch.
So I like that. Nice catch, Shaohua!

Reviewed-by: Minchan Kim <minchan.kim@...il.com>
-- 
Kind regards,
Minchan Kim
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