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Date:	Tue, 7 Oct 2014 09:13:20 -0700
From:	Peter Feiner <pfeiner@...gle.com>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Dr. David Alan Gilbert" <dgilbert@...hat.com>,
	qemu-devel@...gnu.org, KVM list <kvm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	Linux API <linux-api@...r.kernel.org>,
	Andres Lagar-Cavilla <andreslc@...gle.com>,
	Dave Hansen <dave@...1.net>,
	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>,
	Hugh Dickins <hughd@...gle.com>,
	Christopher Covington <cov@...eaurora.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Android Kernel Team <kernel-team@...roid.com>,
	Robert Love <rlove@...gle.com>,
	Dmitry Adamushko <dmitry.adamushko@...il.com>,
	Neil Brown <neilb@...e.de>, Mike Hommey <mh@...ndium.org>,
	Taras Glek <tglek@...illa.com>, Jan Kara <jack@...e.cz>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	Michel Lespinasse <walken@...gle.com>,
	Minchan Kim <minchan@...nel.org>,
	Keith Packard <keithp@...thp.com>,
	"Huangpeng (Peter)" <peter.huangpeng@...wei.com>,
	Isaku Yamahata <yamahata@...inux.co.jp>,
	Anthony Liguori <anthony@...emonkey.ws>,
	Stefan Hajnoczi <stefanha@...il.com>,
	Wenchao Xia <wenchaoqemu@...il.com>,
	Andrew Jones <drjones@...hat.com>,
	Juan Quintela <quintela@...hat.com>
Subject: Re: [PATCH 10/17] mm: rmap preparation for remap_anon_pages

On Tue, Oct 07, 2014 at 05:52:47PM +0200, Andrea Arcangeli wrote:
> I probably grossly overestimated the benefits of resolving the
> userfault with a zerocopy page move, sorry. [...]

For posterity, I think it's worth noting that most expensive aspect of a TLB
shootdown is the interprocessor interrupt necessary to flush other CPUs' TLBs.
On a many-core machine, copying 4K of data looks pretty cheap compared to
taking an interrupt and invalidating TLBs on many cores :-)

> [...] So if we entirely drop the
> zerocopy behavior and the TLB flush of the old page like you
> suggested, the way to keep the userfaultfd mechanism decoupled from
> the userfault resolution mechanism would be to implement an
> atomic-copy syscall. That would work for SIGBUS userfaults too without
> requiring a pseudofd then. It would be enough then to call
> mcopy_atomic(userfault_addr,tmp_addr,len) with the only constraints
> that len must be a multiple of PAGE_SIZE. Of course mcopy_atomic
> wouldn't page fault or call GUP into the destination address (it can't
> otherwise the in-flight partial copy would be visible to the process,
> breaking the atomicity of the copy), but it would fill in the
> pte/trans_huge_pmd with the same strict behavior that remap_anon_pages
> currently has (in turn it would by design bypass the VM_USERFAULT
> check and be ideal for resolving userfaults).
> 
> mcopy_atomic could then be also extended to tmpfs and it would work
> without requiring the source page to be a tmpfs page too without
> having to convert page types on the fly.
> 
> If I add mcopy_atomic, the patch in subject (10/17) can be dropped of
> course so it'd be even less intrusive than the current
> remap_anon_pages and it would require zero TLB flush during its
> runtime (it would just require an atomic copy).

I like this new approach. It will be good to have a single interface for
resolving anon and tmpfs userfaults.

> So should I try to embed a mcopy_atomic inside userfault_write or can
> I expose it to userland as a standalone new syscall? Or should I do
> something different? Comments?

One interesting (ab)use of userfault_write would be that the faulting process
and the fault-handling process could be different, which would be necessary
for post-copy live migration in CRIU (http://criu.org).

Aside from the asthetic difference, I can't think of any advantage in favor of
a syscall.

Peter
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