linux-kernel - Re: [Qemu-devel] [PATCH 10/17] mm: rmap preparation for remap_anon

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Date:	Tue, 7 Oct 2014 15:37:10 +0200
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
Cc:	qemu-devel@...gnu.org, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux-api@...r.kernel.org, Robert Love <rlove@...gle.com>,
	Dave Hansen <dave@...1.net>, Jan Kara <jack@...e.cz>,
	Neil Brown <neilb@...e.de>,
	Stefan Hajnoczi <stefanha@...il.com>,
	Andrew Jones <drjones@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	Michel Lespinasse <walken@...gle.com>,
	Taras Glek <tglek@...illa.com>,
	Juan Quintela <quintela@...hat.com>,
	Hugh Dickins <hughd@...gle.com>,
	Isaku Yamahata <yamahata@...inux.co.jp>,
	Mel Gorman <mgorman@...e.de>,
	Sasha Levin <sasha.levin@...cle.com>,
	Android Kernel Team <kernel-team@...roid.com>,
	"\\\"Dr. David Alan Gilbert\\\"" <dgilbert@...hat.com>,
	"Huangpeng (Peter)" <peter.huangpeng@...wei.com>,
	Andres Lagar-Cavilla <andreslc@...gle.com>,
	Christopher Covington <cov@...eaurora.org>,
	Anthony Liguori <anthony@...emonkey.ws>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Keith Packard <keithp@...thp.com>,
	Wenchao Xia <wenchaoqemu@...il.com>,
	Andy Lutomirski <luto@...capital.net>,
	Minchan Kim <minchan@...nel.org>,
	Dmitry Adamushko <dmitry.adamushko@...il.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Mike Hommey <mh@...ndium.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Feiner <pfeiner@...gle.com>
Subject: Re: [Qemu-devel] [PATCH 10/17] mm: rmap preparation for
 remap_anon_pages

Hi Kirill,

On Tue, Oct 07, 2014 at 02:10:26PM +0300, Kirill A. Shutemov wrote:
> On Fri, Oct 03, 2014 at 07:08:00PM +0200, Andrea Arcangeli wrote:
> > There's one constraint enforced to allow this simplification: the
> > source pages passed to remap_anon_pages must be mapped only in one
> > vma, but this is not a limitation when used to handle userland page
> > faults with MADV_USERFAULT. The source addresses passed to
> > remap_anon_pages should be set as VM_DONTCOPY with MADV_DONTFORK to
> > avoid any risk of the mapcount of the pages increasing, if fork runs
> > in parallel in another thread, before or while remap_anon_pages runs.
> 
> Have you considered triggering COW instead of adding limitation on
> pages' mapcount? The limitation looks artificial from interface POV.

I haven't considered it, mostly because I see it as a feature that it
returns -EBUSY. I prefer to avoid the risk of userland getting a
successful retval but internally the kernel silently behaving
non-zerocopy by mistake because some userland bug forgot to set
MADV_DONTFORK on the src_vma.

COW would be not zerocopy so it's not ok. We get sub 1msec latency for
userfaults through 10gbit and we don't want to risk wasting CPU
caches.

I however considered allowing to extend the strict behavior (i.e. the
feature) later in a backwards compatible way. We could provide a
non-zerocopy beahvior with a RAP_ALLOW_COW flag that would then turn
the -EBUSY error into a copy.

It's also more complex to implement the cow now, so it would make the
code that really matters, harder to review. So it may be preferable to
extend this later in a backwards compatible way with a new
RAP_ALLOW_COW flag.

The current handling the flags is already written in a way that should
allow backwards compatible extension with RAP_ALLOW_*:

#define RAP_ALLOW_SRC_HOLES (1UL<<0)

SYSCALL_DEFINE4(remap_anon_pages,
		unsigned long, dst_start, unsigned long, src_start,
		unsigned long, len, unsigned long, flags)
[..]
	long err = -EINVAL;
[..]
	if (flags & ~RAP_ALLOW_SRC_HOLES)
		return err;
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