Re: [Xen-devel] [PATCH 12 of 18] x86/mm: Make page_lock/unlock() in arch/x86/mm.c externally callable

[Keir Fraser <keir.xen@xxxxxxxxx>]

On 08/12/2011 22:38, "Tim Deegan" <tim@xxxxxxx> wrote:

> At 02:47 -0500 on 08 Dec (1323312447), Andres Lagar-Cavilla wrote:
>> This is necessary for a new consumer of page_lock/unlock to follow in
>> the series.
>> 
>> Signed-off-by: Andres Lagar-Cavilla <andres@xxxxxxxxxxxxxxxx>
> 
> Nak, I'm afraid. 
> 
> These were OK as local functions but if they're going to be made
> generally visible, they need clear comments describing what this
> locking protects and what the discipline is for avoiding deadlocks.
> 
> Perhaps Jan or Keir can supply appropriate words.  The locking was
> introduce in this cset:

It's Jan's work originally, but the basic intention of page_lock is to
serialise pte updates. To aid with this, a page's type cannot change while
its lock is held. No lock nests inside a page lock (not even other page
locks) so there is no deadlock risk.

>     changeset:   17846:09dd5999401b
>     user:        Keir Fraser <keir.fraser@xxxxxxxxxx>
>     date:        Thu Jun 12 18:14:00 2008 +0100
>     files:       xen/arch/x86/domain.c xen/arch/x86/domain_build.c
>     xen/arch/x86/mm.c
>     description:
>     x86: remove use of per-domain lock from page table entry handling
>     
>     This change results in a 5% performance improvement for kernel builds
>     on dual-socket quad-core systems (which is what I used for reference
>     for both 32- and 64-bit). Along with that, the amount of time reported
>     as spent in the kernel gets reduced by almost 25% (the fraction of
>     time spent in the kernel is generally reported significantly higher
>     under Xen than with a native kernel).
>     
>     Signed-off-by: Jan Beulich <jbeulich@xxxxxxxxxx>
>     Signed-off-by: Keir Fraser <keir.fraser@xxxxxxxxxx>
> 
> Tim.



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