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

[Minchan Kim <minchan.kim@gmail.com>]

On Mon, Apr 26, 2010 at 6:49 PM, KAMEZAWA Hiroyuki
<kamezawa.hiroyu@jp.fujitsu.com> wrote:
> On Mon, 26 Apr 2010 18:48:42 +0900
> Minchan Kim <minchan.kim@gmail.com> 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?
>
> 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 ?

Yes. Andrea pointed it out.
I didn't followed the thread whole yet but It seems Mel and Andrea
want to restore anon_vma's atomicity like old than one by one healing.



-- 
Kind regards,
Minchan Kim

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