linux-kernel - Re: [RESEND PATCH v2] compaction: fix COMPACTPAGEFAILED counting

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Message-ID: <AANLkTimzDkU-XqpFRTxB7Y0+q1vfs-o4pd8UrG7HPcNX@mail.gmail.com>
Date:	Sun, 5 Sep 2010 11:51:19 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Lameter <cl@...ux.com>,
	Hugh Dickins <hughd@...gle.com>,
	Andi Kleen <andi@...stfloor.org>, Mel Gorman <mel@....ul.ie>,
	linux-mm <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RESEND PATCH v2] compaction: fix COMPACTPAGEFAILED counting

On Sat, Sep 4, 2010 at 11:04 AM, Wu Fengguang <fengguang.wu@...el.com> wrote:
> On Sat, Sep 04, 2010 at 08:02:27AM +0800, Andrew Morton wrote:
>> On Thu,  2 Sep 2010 23:34:47 +0900
>> Minchan Kim <minchan.kim@...il.com> wrote:
>>
>> > Now update_nr_listpages doesn't have a role. That's because
>> > lists passed is always empty just after calling migrate_pages.
>> > The migrate_pages cleans up page list which have failed to migrate
>> > before returning by aaa994b3.
>> >
>> >  [PATCH] page migration: handle freeing of pages in migrate_pages()
>> >
>> >  Do not leave pages on the lists passed to migrate_pages().  Seems that we will
>> >  not need any postprocessing of pages.  This will simplify the handling of
>> >  pages by the callers of migrate_pages().
>> >
>> > At that time, we thought we don't need any postprocessing of pages.
>> > But the situation is changed. The compaction need to know the number of
>> > failed to migrate for COMPACTPAGEFAILED stat
>> >
>> > This patch makes new rule for caller of migrate_pages to call putback_lru_pages.
>> > So caller need to clean up the lists so it has a chance to postprocess the pages.
>> > [suggested by Christoph Lameter]
>>
>> I'm having trouble predicting what the user-visible effects of this bug
>> might be.  Just an inaccuracy in the COMPACTPAGEFAILED vm event?
>
> Right, it's an accounting fix. Before patch COMPACTPAGEFAILED will
> remain 0 regardless of how many migration failures.
>
> The patch does slightly add dependency for migrate_pages() to return
> error code properly. Before patch, migrate_pages() calls
> putback_lru_pages() regardless of the error code. After patch, the
> migrate_pages() callers will check its return value before calling
> putback_lru_pages().
>
> In current code, the two conditions do seem to match:
>
> "some pages remained in the *from list" == "migrate_pages() returns an error code".

Exactly.
Thanks for the answering instead of me, Wu. :)

> Thanks,
> Fengguang
>



-- 
Kind regards,
Minchan Kim
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