linux-kernel - Re: [PATCH 3/3] mm: adjust rss counters for migration entiries

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Message-Id: <20120111174126.f35e708a.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Wed, 11 Jan 2012 17:41:26 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Konstantin Khlebnikov <khlebnikov@...nvz.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/3] mm: adjust rss counters for migration entiries

On Wed, 11 Jan 2012 12:23:11 +0400
Konstantin Khlebnikov <khlebnikov@...nvz.org> wrote:

> KAMEZAWA Hiroyuki wrote:
> > On Fri, 06 Jan 2012 21:38:56 +0400
> > Konstantin Khlebnikov<khlebnikov@...nvz.org>  wrote:
> >
> >> Memory migration fill pte with migration entry and it didn't update rss counters.
> >> Then it replace migration entry with new page (or old one if migration was failed).
> >> But between this two passes this pte can be unmaped, or task can fork child and
> >> it will get copy of this migration entry. Nobody account this into rss counters.
> >>
> >> This patch properly adjust rss counters for migration entries in zap_pte_range()
> >> and copy_one_pte(). Thus we avoid extra atomic operations on migration fast-path.
> >>
> >> Signed-off-by: Konstantin Khlebnikov<khlebnikov@...nvz.org>
> >
> > It's better to show wheter this is a bug-fix or not in changelog.
> >
> > IIUC, the bug-fix is the 1st harf of this patch + patch [2/3].
> > Your new bug-check code is in patch[1/3] and 2nd half of this patch.
> >
> 
> No, there only one new bug-check in 1st patch, this is non-fatal warning.
> I didn't hide this check under CONFIG_VM_DEBUG because it rather small and
> rss counters covers whole page-table management, this is very good invariant.
> Currently I can trigger this warning only on this rare race -- extremely loaded
> memory compaction catches this every several seconds.
> 
> 1/3 bug-check
> 2/3 fix preparation
> 3/3 bugfix in two places:
>      do rss++ in copy_one_pte()
>      do rss-- in zap_pte_range()
> 

Hmm, ok, I read wrong.

So, I think you should post the patch with [BUGFIX] and
report 'what happens' and 'what is the bug' , 'what you fixed' explicitly.

As...
==
  This patch series fixes per-mm rss counter accounting bug. When pages are
  heavily migrated, the rss counters will go wrong by fork() and unmap()
  because they ignores migration_pte_entries.
  This rarelly happens but will make rss counter incorrect.

  This seires of patches will fix the issue by adding proper accounting of
  migration_pte_entries in unmap() and fork(). This series includes
  bug check code, too.
==

If [BUGFIX], people will have more interests.

Anyway, thank you for bugfix.

-Kame

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