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

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Message-Id: <20100426184908.3c277568.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Mon, 26 Apr 2010 18:49:08 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Minchan Kim <minchan.kim@...il.com>
Cc:	Mel Gorman <mel@....ul.ie>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	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 18:48:42 +0900
Minchan Kim <minchan.kim@...il.com> wrote:

> On Mon, Apr 26, 2010 at 6:28 PM, KAMEZAWA Hiroyuki
> <kamezawa.hiroyu@...fujitsu.com> wrote:
> > 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.
> >
> > Ok, reproduced. Here is status in my test + printk().
> >
> >  * A race doesn't seem to happen if swap=off.
> >    I need to swapon to cause the bug
> 
> FYI,
> 
> Do you have a swapon/off bomb test?

No. Just running test under swapoff, and running the same test after swapon.


> When I saw your mail, I feel it might be culprit.
> 
> http://lkml.org/lkml/2010/4/22/762.
> 
> It is just guessing. I don't have a time to look into, now.
> 
Hmm. BTW.

==
static int expand_downwards(struct vm_area_struct *vma,
                                   unsigned long address)
{
   ....
       /* Somebody else might have raced and expanded it already */
        if (address < vma->vm_start) {
                unsigned long size, grow;

                size = vma->vm_end - address;
                grow = (vma->vm_start - address) >> PAGE_SHIFT;

                error = acct_stack_growth(vma, size, grow);
                if (!error) {
                        vma->vm_start = address;
                        vma->vm_pgoff -= grow;
                }
        }
==	

I feel this part needs care. No ?

Thanks,
-Kmae



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