linux-kernel - Re: [PATCH 1/2] x86/mm: maintain a percpu "in get_user_pages

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Date:	Fri, 27 Mar 2009 22:01:11 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Avi Kivity <avi@...hat.com>
CC:	Nick Piggin <npiggin@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux Memory Management List <linux-mm@...ck.org>,
	the arch/x86 maintainers <x86@...nel.org>
Subject: Re: [PATCH 1/2] x86/mm: maintain a percpu "in get_user_pages_fast"
 flag

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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