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Subject: Re: [patch 4/5]thp: correct order in lru list for split huge page
From: Shaohua Li <shaohua.li@intel.com>
To: Minchan Kim <minchan.kim@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        "aarcange@redhat.com" <aarcange@redhat.com>,
        Hugh Dickins <hughd@google.com>, Rik van Riel <riel@redhat.com>,
        mel <mel@csn.ul.ie>,
        KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
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Date: Fri, 28 Oct 2011 13:08:58 +0800
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On Fri, 2011-10-28 at 07:19 +0800, Minchan Kim wrote:
> 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@intel.com>
> 
> In case of a, it doesn't matter ordering of subpages.
> As a huge page, age of sub pages are same.
It does matter. Hugepage is split first and then reclaim. if page, page
+HPAGE_PMD_NR-1 is reclaimed, you can't get an order-1 page. but if
page, page+1 is reclaimed, you can.

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