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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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