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    SubjectRe: long sleep_on_page delays writing to slow storage
    On Wed, Nov 09, 2011 at 07:06:46PM +0100, Andrea Arcangeli wrote:
    > On Wed, Nov 09, 2011 at 05:52:01PM +0000, Mel Gorman wrote:
    > > -#define alloc_pages_vma(gfp_mask, order, vma, addr, node) \
    > > - alloc_pages(gfp_mask, order)
    > > +#define alloc_pages_vma(gfp_mask, order, vma, addr, node, drop_mmapsem) \
    > > + ({ \
    > > + if (drop_mmapsem) \
    > > + up_read(&vma->vm_mm->mmap_sem); \
    > > + alloc_pages(gfp_mask, order); \
    > > + })
    >
    > I wouldn't change alloc_pages_vma. I think it's better to add and have
    > that called only by khugepaged:
    >
    > alloc_pages_vma_up_read(gfp_mask, order, vma, addr, node)
    > {
    > __alloc_pages_vma(gfp_mask, order, vma, addr, node, true);
    > }
    >
    > alloc_pages_vma(gfp_mask, order, vma, addr, node)
    > {
    > __alloc_pages_vma(gfp_mask, order, vma, addr, node, false);
    > }
    >
    > I wonder if a change like this would be enough?
    >
    > sync_migration = !(gfp_mask & __GFP_NO_KSWAPD);
    >
    > But even if hidden in a new function, the main downside overall is the
    > fact we'll pass one more var through the stack of fast paths.
    >
    > Johannes I recall you reported this too and Mel suggested the above
    > change, did it help in the end?

    Yes, it completely fixed the latency problem.

    That said, I haven't looked at the impact on the THP success rate, but
    a regression there is probably less severe than half-minute-stalls in
    interactive applications.


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