Re: [PATCH 1/2] x86/mm: maintain a percpu "in get_user_pages_fast"flag

[Jeremy Fitzhardinge]

Avi Kivity wrote:
> Jeremy Fitzhardinge wrote:
> > get_user_pages_fast() relies on cross-cpu tlb flushes being a barrier
> > between clearing and setting a pte, and before freeing a pagetable page.
> > It usually does this by disabling interrupts to hold off IPIs, but
> > some tlb flush implementations don't use IPIs for tlb flushes, and
> > must use another mechanism.
> >
> > In this change, add in_gup_cpumask, which is a cpumask of cpus currently
> > performing a get_user_pages_fast traversal of a pagetable.  A cross-cpu
> > tlb flush function can use this to determine whether it should hold-off
> > on the flush until the gup_fast has finished.
> >
> > @@ -255,6 +260,10 @@ int get_user_pages_fast(unsigned long start, int 
> > nr_pages, int write,
> >      * address down to the the page and take a ref on it.
> >      */
> >     local_irq_disable();
> > +
> > +    cpu = smp_processor_id();
> > +    cpumask_set_cpu(cpu, in_gup_cpumask);
> > +
>
> This will bounce a cacheline, every time.  Please wrap in CONFIG_XEN 
> and skip at runtime if Xen is not enabled.

Every time?  Only when running successive gup_fasts on different cpus, 
and only twice per gup_fast. (What's the typical page count?  I see that 
kvm and lguest are page-at-a-time users, but presumably direct IO has 
larger batches.)

Alternatively, it could have per-cpu flags and the other side could 
construct the mask (I originally had that, but this was simpler).

   J
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