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. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
