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

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Date:	Sat, 4 Sep 2010 10:04:53 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Minchan Kim <minchan.kim@...il.com>,
	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 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".

Thanks,
Fengguang
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