Re: [PATCH] mm: exempt pcp alloc from watermarks

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Wed, 2006-09-20 at 01:10 +1000, Nick Piggin wrote:
> Peter Zijlstra wrote:
> > On Wed, 2006-09-20 at 00:35 +1000, Nick Piggin wrote:
> > 
> 
> >>Thanks for the patch! I have a slight preference for the following
> >>version, which speculatively tests pcp->count without disabling
> >>interrupts (the chance of being preempted or scheduled in this
> >>window is basically the same as the chance of being preempted after
> >>checking watermarks). What do you think?
> > 
> > 
> > The race here allows to wrongly bypass the watermark check. My version
> > raced the other way about, where you could find a non empty pcp where an
> > empty one was otherwise expected.
> 
> I really doubt it matters. You could be preempted after that check
> anyway, and by the time you return the previous watermark check is
> meaningless.
> 
> If we really want to be strict about watermark checks, it has to be
> done with the zone lock held, no other option. I doubt anybody
> bothered, because the watermarks (even PF_MEMALLOC pool) are all
> heuristics anyway and it is a better idea to keep fastpath code fast.

Yes, you're absolutely right. I forgot to look at the bigger picture.

Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>

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