WARNING - OLD ARCHIVES

This is an archived copy of the Xen.org mailing list, which we have preserved to ensure that existing links to archives are not broken. The live archive, which contains the latest emails, can be found at http://lists.xen.org/
   
 
 
Xen 
 
Home Products Support Community News
 
   
 

xen-devel

Re: [Xen-devel] Fwd: Re: struct page field arrangement

To: "Keir Fraser" <keir@xxxxxxxxxxxxx>
Subject: Re: [Xen-devel] Fwd: Re: struct page field arrangement
From: "Jan Beulich" <jbeulich@xxxxxxxxxx>
Date: Fri, 16 Mar 2007 12:54:08 +0000
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx
Delivery-date: Fri, 16 Mar 2007 05:52:18 -0700
Envelope-to: www-data@xxxxxxxxxxxxxxxxxx
In-reply-to: <C2203D35.BA7D%keir@xxxxxxxxxxxxx>
List-help: <mailto:xen-devel-request@lists.xensource.com?subject=help>
List-id: Xen developer discussion <xen-devel.lists.xensource.com>
List-post: <mailto:xen-devel@lists.xensource.com>
List-subscribe: <http://lists.xensource.com/cgi-bin/mailman/listinfo/xen-devel>, <mailto:xen-devel-request@lists.xensource.com?subject=subscribe>
List-unsubscribe: <http://lists.xensource.com/cgi-bin/mailman/listinfo/xen-devel>, <mailto:xen-devel-request@lists.xensource.com?subject=unsubscribe>
References: <C2203A05.BA74%keir@xxxxxxxxxxxxx> <C2203D35.BA7D%keir@xxxxxxxxxxxxx>
Sender: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
>>> Keir Fraser <keir@xxxxxxxxxxxxx> 16.03.07 13:25 >>>
>On 16/3/07 12:11, "Keir Fraser" <keir@xxxxxxxxxxxxx> wrote:
>
>>> page_referenced_one() in mm/rmap.c). If this happens when
>>> xen_pgd_unpin() has already passed the respective pte page, but
>>> mm_walk() hasn't reached the page, yet, the update will fail (if done
>>> directly, ptwr will no pick this up, and if done through a hypercall, the
>>> call would fail, likely producing a BUG()).
>> 
>> What kind of stress test did you run? I was expecting that unpin would be
>> okay because we only call mm_unpin() from _arch_exit_mmap() if the mm_count
>> is 1 (which I believe means the mm is not active in any task).

newburn on machines with not too much (<= 2G) memory.

>And actually the pinning happens on activate_mm() in most cases, which I
>would expect to be 'early enough' since noone can run on the mm before that?
>
>If you've managed to provoke bugs then that's very interesting (and scary)!
>
>I suppose if I understand the rmap case correctly, we're still susceptible
>to the paging kernel thread trying to page things out at any time? Is that
>what you think you've been seeing go wrong?

Yes, somewhere in that area. From the data I have (page fault on the
page table write in ptep_clear_flush_young(), with the page table dump
showing the page to be writeable and present) I can only conclude that
the race is with the unpin path (otherwise I should see the page being
write protected), while the vm scan tries to recover memory at the same
time, and since this scan is scanning zones, not mm-s, the references to
the mm-s are being obtained from struct page -> vma -> mm (i.e. the
mm-s' use counts don't matter here).

Jan

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel