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SubjectRe: [PATCH 1/2] x86/mm: maintain a percpu "in get_user_pages_fast" flag
Jeremy Fitzhardinge wrote:
>>> @@ -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.)

Databases will often issue I/Os of 1 or 2 pages. But not regressing kvm
should be sufficient motivation.


--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.



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