linux-kernel - Re: [BUGFIX][mm][PATCH] fix migration race in rmap

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Date:	Mon, 26 Apr 2010 11:53:47 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	Mel Gorman <mel@....ul.ie>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"minchan.kim@...il.com" <minchan.kim@...il.com>,
	Christoph Lameter <cl@...ux.com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [BUGFIX][mm][PATCH] fix migration race in rmap_walk

On Mon, 26 Apr 2010 08:49:01 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> wrote:

> On Sat, 24 Apr 2010 11:43:24 +0100
> Mel Gorman <mel@....ul.ie> wrote:

> > It looks nice but it still broke after 28 hours of running. The
> > seq-counter is still insufficient to catch all changes that are made to
> > the list. I'm beginning to wonder if a) this really can be fully safely
> > locked with the anon_vma changes and b) if it has to be a spinlock to
> > catch the majority of cases but still a lazy cleanup if there happens to
> > be a race. It's unsatisfactory and I'm expecting I'll either have some
> > insight to the new anon_vma changes that allow it to be locked or Rik
> > knows how to restore the original behaviour which as Andrea pointed out
> > was safe.
> > 
> Ouch. Hmm, how about the race in fork() I pointed out ?
> 
Forget this. Sorry for noise.

==
This is a memo for myself.

*) at fork, when copying a vma for file, vma_prio_tree_add() is called
   before copying page tables.
   There are several patterns.

Assume tasks named as t1,t2,t3,t4,t5 and their own vmas v1,v2,v3,v4,v5 which map
a range in address spaces.

(a) t1 forks t2.
   v1 is in prio_tree, v2 for t2 will be pointed by ->head pointer.

   \
    v1  --(head)---> v2 
   /  \
  ?    ?

  vma_prio_tree_foreach() order : v1->v2.


(b) after (a), t2 forks t3. (list_add() is used.)
 
    \
     v1 --(head)--> v2 ->(list.next)->v3
    /  \
   ?    ?

   vma_prio_tree_foreach() order : v1->v2->v3

(c) after (b), t1 forks t4.

    \
     v1 --(head)--> v2 ->(list.next)->v3->v4
    /  \              
   ?    ?

    vma_prio_tree_foreach() order : v1->v2->v3->v4

(d) after (c), t4 forks t5.

    \
     v1 --(head)--> v2 ->(list.next)->v3->v4->v5
    /  \               
   ?    ?
    vma_prio_tree_foreach() order : v1->v2->v3->v4->v5

(e) after (c), t3 forks t5.
    \
     v1 --(head)--> v2 ->(list.next)->v3->v5->v4-
    /  \               
   ?    ?
    vma_prio_tree_foreach() order : v1->v2->v3->v5->v4

.....in any case, it seems vma_prio_tree_foreach() finds
the parent's vma 1st.

Thx,
-Kame















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