linux-kernel - Re: RFC: get_user_pages_locked|unlocked to leverage VM_FAULT

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Date:	Thu, 2 Oct 2014 14:31:17 +0200
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Andres Lagar-Cavilla <andreslc@...gle.com>,
	Gleb Natapov <gleb@...nel.org>,
	Radim Krcmar <rkrcmar@...hat.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
	Andy Lutomirski <luto@...capital.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Sasha Levin <sasha.levin@...cle.com>,
	Jianyu Zhan <nasa4836@...il.com>,
	Paul Cassella <cassella@...y.com>,
	Hugh Dickins <hughd@...gle.com>,
	Peter Feiner <pfeiner@...gle.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	"Dr. David Alan Gilbert" <dgilbert@...hat.com>
Subject: Re: RFC: get_user_pages_locked|unlocked to leverage VM_FAULT_RETRY

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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