linux-kernel - Re: [PATCH v3 2/2] migrate: add migrate_entry_wait

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Date:	Fri, 31 May 2013 15:46:24 -0400
From:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-mm@...ck.org, Mel Gorman <mgorman@...e.de>,
	Andi Kleen <andi@...stfloor.org>,
	Michal Hocko <mhocko@...e.cz>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/2] migrate: add migrate_entry_wait_huge()

> That changelog is not suitable for a -stable patch.  People who
> maintain and utilize the stable trees need to know in some detail what
> is the end-user impact of the patch (or, equivalently, of the bug
> which the patch fixes).

OK. So, could you insert the following sentence?

On Fri, May 31, 2013 at 12:30:03PM -0700, Andrew Morton wrote:
> On Tue, 28 May 2013 15:52:51 -0400 Naoya Horiguchi <n-horiguchi@...jp.nec.com> wrote:
> 
> > When we have a page fault for the address which is backed by a hugepage
> > under migration, the kernel can't wait correctly and do busy looping on
> > hugepage fault until the migration finishes.

As a result, users who try to kick hugepage migration (via soft offlining,
for example) occasionally experience long delay or soft lockup.

Thanks,
Naoya

> > This is because pte_offset_map_lock() can't get a correct migration entry
> > or a correct page table lock for hugepage.
> > This patch introduces migration_entry_wait_huge() to solve this.
> > 
> > Note that the caller, hugetlb_fault(), gets the pointer to the "leaf"
> > entry with huge_pte_offset() inside which all the arch-dependency of
> > the page table structure are. So migration_entry_wait_huge() and
> > __migration_entry_wait() are free from arch-dependency.
> > 
> > ChangeLog v3:
> >  - use huge_pte_lockptr
> > 
> > ChangeLog v2:
> >  - remove dup in migrate_entry_wait_huge()
> > 
> > Signed-off-by: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
> > Reviewed-by: Rik van Riel <riel@...hat.com>
> > Cc: stable@...r.kernel.org # 2.6.35
> 
> That changelog is not suitable for a -stable patch.  People who
> maintain and utilize the stable trees need to know in some detail what
> is the end-user impact of the patch (or, equivalently, of the bug
> which the patch fixes).
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