Re: RFC: get_user_pages_locked|unlocked to leverage VM_FAULT_RETRY

[Andrea Arcangeli <aarcange@redhat.com>]

On Wed, Oct 01, 2014 at 05:36:11PM +0200, Peter Zijlstra wrote:
> For all these and the other _fast() users, is there an actual limit to
> the nr_pages passed in? Because we used to have the 64 pages limit from
> DIO, but without that we get rather long IRQ-off latencies.

Ok, I would tend to think this is an issue to solve in gup_fast
implementation, I wouldn't blame or modify the callers for it.

I don't think there's anything that prevents gup_fast to enable irqs
after certain number of pages have been taken, nop; and disable the
irqs again.

If the TLB flush runs in parallel with gup_fast the result is
undefined anyway so there's no point to wait all pages to be taken
before letting the TLB flush go through. All it matters is that
gup_fast don't take pages that have been invalidated after the
tlb_flush returns on the other side. So I don't see issues in
releasing irqs and be latency friendly inside gup_fast fast path loop.

In fact gup_fast should also cond_resched() after releasing irqs, it's
not just an irq latency matter.

I could fix x86-64 for it in the same patchset unless somebody sees a
problem in releasing irqs inside the gup_fast fast path loop.

__gup_fast is an entirely different beast and that needs the callers to
be fixed but I didn't alter its callers.

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