Re: [Xen-devel] [PATCH] x86: fix preemptable page type handling(v2)

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

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



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