Re: [BUGFIX][mm][PATCH] fix migration race in rmap_walk

[KAMEZAWA Hiroyuki]

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

> On Sat, 24 Apr 2010 11:43:24 +0100
> Mel Gorman <mel@xxxxxxxxx> 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.
 * Before unmap, mapcount=1, SwapCache for anonymous memory.
   old page's flag was SWAPCACHE, Active, Uptodate, Referenced, Locked.
 * After remap, mapcount=0, return code=0.
   new page's flag after remap was SwapCache, Active, Dirty, Uptodate, Referenced.

(Hmm, dirty bit can be added by try_to_unamp().)

-Kame



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