lkml.org 
[lkml]   [2010]   [Apr]   [26]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [BUGFIX][mm][PATCH] fix migration race in rmap_walk
On Mon, Apr 26, 2010 at 06:48:42PM +0900, Minchan Kim wrote:
> On Mon, Apr 26, 2010 at 6:28 PM, KAMEZAWA Hiroyuki
> <kamezawa.hiroyu@jp.fujitsu.com> wrote:
> > On Mon, 26 Apr 2010 08:49:01 +0900
> > KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
> >
> >> On Sat, 24 Apr 2010 11:43:24 +0100
> >> Mel Gorman <mel@csn.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?
> 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.
>

I haven't tried a swapon/off test but that patch is certainly important
and closes an important race. A fork-heavy test will routinely hit the
problem and applying the patch makes it very difficult to reproduce the
problem. I've added it to my stack while I continue trying to pin down
when the VMA-changes make a difference.

I'm relooking at the seq counter approach. It appears to very rare the logic
is actually triggered so reproducing is a problem. I'm still not convinced
that just locking anon_vma is not the answer there. If it locks and as
expand_downwards already locks and with the fork-based patch, I think the
races might be closed but I'm not 100% certain yet.

--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2010-04-26 13:39    [W:0.038 / U:0.076 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site