Re: RFC: get_user_pages_locked|unlocked to leverage VM_FAULT_RETRY

[Peter Zijlstra]

On Thu, Oct 02, 2014 at 02:31:17PM +0200, Andrea Arcangeli wrote:
> 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.
> 

Agreed, I once upon a time had a patch set converting the 2 (x86 and
powerpc) gup_fast implementations at the time, but somehow that never
got anywhere.

Just saying we should probably do that before we add callers with
unlimited nr_pages.
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