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

[Xen-devel] Re: [Xen-changelog] [xen-unstable] kunmap_atomic() must zap

To: xen-devel@xxxxxxxxxxxxxxxxxxx
Subject: [Xen-devel] Re: [Xen-changelog] [xen-unstable] kunmap_atomic() must zap the PTE to avoid dangling references
From: Isaku Yamahata <yamahata@xxxxxxxxxxxxx>
Date: Fri, 7 Jul 2006 21:38:13 +0900
Cc: xen-ia64-devel@xxxxxxxxxxxxxxxxxxx
Delivery-date: Fri, 07 Jul 2006 05:39:11 -0700
Envelope-to: www-data@xxxxxxxxxxxxxxxxxx
In-reply-to: <E1FwL6V-0007Fj-9w@xxxxxxxxxxxxxxxxxxxxx>
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: <E1FwL6V-0007Fj-9w@xxxxxxxxxxxxxxxxxxxxx>
Sender: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
User-agent: Mutt/1.4.2.1i
On Fri, Jun 30, 2006 at 03:40:22PM +0000, Xen patchbot-unstable wrote:

> # HG changeset patch
> # User kfraser@xxxxxxxxxxxxxxxxxxxxxxx
> # Node ID 4260eb8c08740de0000081c61a6237ffcb95b2d5
> # Parent  5fa6c1723e08d6395fecc427f372d008006b2b4c
> kunmap_atomic() must zap the PTE to avoid dangling references
> when attempting to free memory back to Xen. We can implement
> something more efficient in future.
> 
> Also add debug print message if guest tries to free 'in use'
> memory. We'll make it a real guest-visible error in future.
...
> diff -r 5fa6c1723e08 -r 4260eb8c0874 xen/common/memory.c
> --- a/xen/common/memory.c     Wed Jun 28 17:56:34 2006 +0100
> +++ b/xen/common/memory.c     Wed Jun 28 18:17:41 2006 +0100
> @@ -169,6 +169,15 @@ guest_remove_page(
>              
>      if ( test_and_clear_bit(_PGC_allocated, &page->count_info) )
>          put_page(page);
> +
> +    if ( unlikely((page->count_info & PGC_count_mask) != 1) )
> +    {
> +        /* We'll make this a guest-visible error in future, so take heed! */
> +        DPRINTK("Dom%d freeing in-use page %lx (pseudophys %lx):"
> +                " count=%x type=%lx\n",
> +                d->domain_id, mfn, get_gpfn_from_mfn(mfn),
> +                page->count_info, page->u.inuse.type_info);
> +    }
>  
>      guest_physmap_remove_page(d, gmfn, mfn);

The above check results in false positive on Xen/IA64
because a page 'in use' has reference count 2 on Xen/IA64.
- a page is assigned to guest domain's psudo physical address space.
  This is decremented by guest_physmap_remove_page()
- a page is allocated for a domain.
  This is decremented by the following put_page()

Work around is needed.
I think the option B. is better, but I'm not sure.
Which option is better?

A. just #ifdef.
   poor solution.
B. define an arch-specific constant like PGC_removable
C. define an arch-specific checking macro or function like
   is_page_removable().
D. move the check under guest_physical_remove_page().
   Probably this option isn't good because we may want to detect an error
   before doing complex thing by guest_physimap_remove_page().
E. others.

-- 
yamahata

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

<Prev in Thread] Current Thread [Next in Thread>
  • [Xen-devel] Re: [Xen-changelog] [xen-unstable] kunmap_atomic() must zap the PTE to avoid dangling references, Isaku Yamahata <=