From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from eggs.gnu.org ([2001:4830:134:3::10]:54842)
	by lists.gnu.org with esmtp (Exim 4.71)
	(envelope-from <pbonzini@redhat.com>) id 1XbYMS-0004F9-Oy
	for qemu-devel@nongnu.org; Tue, 07 Oct 2014 13:15:50 -0400
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71)
	(envelope-from <pbonzini@redhat.com>) id 1XbYMM-00018X-Ei
	for qemu-devel@nongnu.org; Tue, 07 Oct 2014 13:15:44 -0400
Received: from mx1.redhat.com ([209.132.183.28]:38672)
	by eggs.gnu.org with esmtp (Exim 4.71)
	(envelope-from <pbonzini@redhat.com>) id 1XbYML-00017i-Uu
	for qemu-devel@nongnu.org; Tue, 07 Oct 2014 13:15:38 -0400
Message-ID: <54341F77.6060400@redhat.com>
Date: Tue, 07 Oct 2014 19:14:31 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
MIME-Version: 1.0
References: <1412356087-16115-1-git-send-email-aarcange@redhat.com>
	<1412356087-16115-11-git-send-email-aarcange@redhat.com>
	<CA+55aFx++R42L75ooE=Fmaem73=V=q7f6pYTcALxgrA1y98G-A@mail.gmail.com>
	<20141006085540.GD2336@work-vm> <20141006164156.GA31075@redhat.com>
	<CA+55aFxAOYBny+QwXfkPy-P3rs-RPr5SLYLcPNBiFO3waBXtQA@mail.gmail.com>
	<20141007170731.GO2404@work-vm>
In-Reply-To: <20141007170731.GO2404@work-vm>
Content-Type: text/plain; charset=windows-1252
Content-Transfer-Encoding: 7bit
Subject: Re: [Qemu-devel] [PATCH 10/17] mm: rmap preparation for
	remap_anon_pages
List-Id: <qemu-devel.nongnu.org>
List-Unsubscribe: <https://lists.nongnu.org/mailman/options/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=unsubscribe>
List-Archive: <http://lists.nongnu.org/archive/html/qemu-devel>
List-Post: <mailto:qemu-devel@nongnu.org>
List-Help: <mailto:qemu-devel-request@nongnu.org?subject=help>
List-Subscribe: <https://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=subscribe>
To: "Dr. David Alan Gilbert" <dgilbert@redhat.com>, Linus Torvalds <torvalds@linux-foundation.org>
Cc: Robert Love <rlove@google.com>, Dave Hansen <dave@sr71.net>, Jan Kara <jack@suse.cz>, KVM list <kvm@vger.kernel.org>, Neil Brown <neilb@suse.de>, Stefan Hajnoczi <stefanha@gmail.com>, qemu-devel@nongnu.org, linux-mm <linux-mm@kvack.org>, KOSAKI Motohiro <kosaki.motohiro@gmail.com>, Michel Lespinasse <walken@google.com>, Andrea Arcangeli <aarcange@redhat.com>, Taras Glek <tglek@mozilla.com>, Andrew Jones <drjones@redhat.com>, Juan Quintela <quintela@redhat.com>, Hugh Dickins <hughd@google.com>, Isaku Yamahata <yamahata@valinux.co.jp>, Mel Gorman <mgorman@suse.de>, Sasha Levin <sasha.levin@oracle.com>, Android Kernel Team <kernel-team@android.com>, "Huangpeng (Peter)" <peter.huangpeng@huawei.com>, Andres Lagar-Cavilla <andreslc@google.com>, Christopher Covington <cov@codeaurora.org>, Anthony Liguori <anthony@codemonkey.ws>, Keith Packard <keithp@keithp.com>, Wenchao Xia <wenchaoqemu@gmail.com>, Linux API <linux-api@vger.kernel.org>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, Andy Lutomirski <luto@amacapital.net>, Minchan Kim <minchan@kernel.org>, Dmitry Adamushko <dmitry.adamushko@gmail.com>, Johannes Weiner <hannes@cmpxchg.org>, Mike Hommey <mh@glandium.org>, Andrew Morton <akpm@linux-foundation.org>, Peter Feiner <pfeiner@google.com>

Il 07/10/2014 19:07, Dr. David Alan Gilbert ha scritto:
>> > 
>> > So I'd *much* rather have a "write()" style interface (ie _copying_
>> > bytes from user space into a newly allocated page that gets mapped)
>> > than a "remap page" style interface
> Something like that might work for the postcopy case; it doesn't work
> for some of the other uses that need to stop a page being changed by the
> guest, but then need to somehow get a copy of that page internally to QEMU,
> and perhaps provide it back later.

I cannot parse this.  Which uses do you have in mind?  Is it for
QEMU-specific or is it for other applications of userfaults?

As long as the page is atomically mapped, I'm not sure what the
difference from remap_anon_pages are (as far as the destination page is
concerned).  Are you thinking of having userfaults enabled on the source
as well?

Paolo

> remap_anon_pages worked for those cases
> as well; I can't think of another current way of doing it in userspace.