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] [PATCH] x86: fix preemptable page type handling(v2)

To: Jan Beulich <jbeulich@xxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH] x86: fix preemptable page type handling(v2)
From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Date: Thu, 30 Oct 2008 15:00:20 +0000
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx
Delivery-date: Thu, 30 Oct 2008 08:00:46 -0700
Envelope-to: www-data@xxxxxxxxxxxxxxxxxxx
In-reply-to: <4909D910.76E4.0078.0@xxxxxxxxxx>
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/mailman/listinfo/xen-devel>, <mailto:xen-devel-request@lists.xensource.com?subject=subscribe>
List-unsubscribe: <http://lists.xensource.com/mailman/listinfo/xen-devel>, <mailto:xen-devel-request@lists.xensource.com?subject=unsubscribe>
Sender: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
Thread-index: Ack6oDoOeJ+0fqaTEd23kwAX8io7RQ==
Thread-topic: [Xen-devel] [PATCH] x86: fix preemptable page type handling(v2)
User-agent: Microsoft-Entourage/11.4.0.080122
On 30/10/08 14:56, "Jan Beulich" <jbeulich@xxxxxxxxxx> wrote:

>> get_page() across all __put_page() operations and then skipping the
>> put_page() in some cases, for example, seems inefficient and also makes the
>> code less clear. Why would you do it that way?
> 
> Because I can't handle a failure of get_page() (due to count overflow)
> once I'm past the put()s.

We could make get_page() always leave a few available references, and then
define a new get_page() variant which:
 A. Does not check the owner
 B. Knows that it is incrementing a non-zero count
 C. Is able to use the few references never claimed by get_page().

This would be basically a small cmpxchg() loop. Since it's only executed
before setting PGT_partial we know that only one such special reference is
needed per page and we could just make get_page() leave headroom of one
reference.

Simpler code and faster code?

 -- Keir



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