linux-kernel - Re: [PATCH 3/5] track the root (oldest) anon

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Message-Id: <20100514090458.acaedb48.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Fri, 14 May 2010 09:04:58 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Rik van Riel <riel@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mel@....ul.ie>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Minchan Kim <minchan.kim@...il.com>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 3/5] track the root (oldest) anon_vma

On Wed, 12 May 2010 22:25:15 -0400
Rik van Riel <riel@...hat.com> wrote:

> On 05/12/2010 08:38 PM, KAMEZAWA Hiroyuki wrote:
> > On Wed, 12 May 2010 13:39:58 -0400
> > Rik van Riel<riel@...hat.com>  wrote:
> >
> >> Subject: track the root (oldest) anon_vma
> >>
> >> Track the root (oldest) anon_vma in each anon_vma tree.   Because we only
> >> take the lock on the root anon_vma, we cannot use the lock on higher-up
> >> anon_vmas to lock anything.  This makes it impossible to do an indirect
> >> lookup of the root anon_vma, since the data structures could go away from
> >> under us.
> >>
> >> However, a direct pointer is safe because the root anon_vma is always the
> >> last one that gets freed on munmap or exit, by virtue of the same_vma list
> >> order and unlink_anon_vmas walking the list forward.
> >>
> >> Signed-off-by: Rik van Riel<riel@...hat.com>
> >
> >
> > Acked-by: KAMEZAWA Hiroyuki<kamezawa.hiroyu@...fujitsu.com>
> >
> > I welcome this. Thank you!
> >
> > Reading 4/5, I felt I'm grad if you add a Documentation or very-precise-comment
> > about the new anon_vma rules and the _meaning_ of anon_vma_root_lock.
> > I cannot fully convice myself that I understand them all.
> 
> Please send me a list of all the questions that come up
> when you read the patches, and I'll prepare a patch 6/5
> with just documentation :)
> 

0. Why it's dangerous to take vma->anon_vma->lock ?

1. What kinds of anon_vmas we'll found in
     page->mapping => anon_vma->head and avc->same_anon_vma ?
   IOW, what kinds of avc->vmas will see when we walk anon_vma->head.

2. Why we have to walk from the root ?

3. What anon_vma_lock guards, actually ?


etc....the facts which is unclear for guys who are not involved in this fix.
Preparing some explanation seems to be kindly rather than "plz ask google"

Bye.
-Kame


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