linux-kernel - Re: [PATCH 1/3] mm,migration: During fork(), wait for migration to end if migration PTE is encountered

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Message-Id: <20100428092802.816e2716.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Wed, 28 Apr 2010 09:28:02 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	Mel Gorman <mel@....ul.ie>, Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Minchan Kim <minchan.kim@...il.com>,
	Christoph Lameter <cl@...ux.com>,
	Rik van Riel <riel@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 1/3] mm,migration: During fork(), wait for migration to
 end if migration PTE is encountered

On Wed, 28 Apr 2010 02:19:11 +0200
Andrea Arcangeli <aarcange@...hat.com> wrote:

> On Wed, Apr 28, 2010 at 02:18:21AM +0200, Andrea Arcangeli wrote:
> > On Wed, Apr 28, 2010 at 08:52:03AM +0900, KAMEZAWA Hiroyuki wrote:
> > > I already explained this doesn't happend and said "I'm sorry".
> > 
> > Oops I must have overlooked it sorry! I just seen the trace quoted in
> > the comment of the patch and that at least would need correction
> > before it can be pushed in mainline, or it creates huge confusion to
> > see a reverse trace for CPU A for an already tricky piece of code.
> > 
> > > But considering maintainance, it's not necessary to copy migration ptes
> > > and we don't have to keep a fundamental risks of migration circus.
> > > 
> > > So, I don't say "we don't need this patch."
> > 
> > split_huge_page also has the same requirement and there is no bug to
> > fix, so I don't see why to make special changes for just migrate.c
> > when we still have to list_add_tail for split_huge_page.
> > 
> > Furthermore this patch isn't fixing anything in any case and it looks
> > a noop to me. If the order ever gets inverted, and process2 ptes are
> > scanned before process1 ptes in the rmap_walk, sure the
> > copy-page-tables will break and stop until the process1 rmap_walk will
> > complete, but that is not enough! You have to repeat the rmap_walk of
> > process1 if the order ever gets inverted and this isn't happening in
>   ^^^^^^^2

why we have to remove migration_pte by rmap_walk() which doesnt' exist ?

Anyway, I agree there are no oops. But there are risks because migration is
a feature which people don't tend to take care of (as memcg ;)
I like conservative approach for this kind of features.

Thanks,
-Kame










 

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