linux-kernel - Re: [PATCH 00/17] RFC: userfault v2

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Date:	Fri, 21 Nov 2014 15:19:28 +0800
From:	zhanghailiang <zhang.zhanghailiang@...wei.com>
To:	Andrea Arcangeli <aarcange@...hat.com>
CC:	"Dr. David Alan Gilbert" <dgilbert@...hat.com>,
	<qemu-devel@...gnu.org>, <kvm@...r.kernel.org>,
	<linux-kernel@...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>,
	Peter Feiner <pfeiner@...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 00/17] RFC: userfault v2

On 2014/11/21 1:38, Andrea Arcangeli wrote:
> Hi,
>
> On Thu, Nov 20, 2014 at 10:54:29AM +0800, zhanghailiang wrote:
>> Yes, you are right. This is what i really want, bypass all non-present faults
>> and only track strict wrprotect faults. ;)
>>
>> So, do you plan to support that in the userfault API?
>
> Yes I think it's good idea to support wrprotect/COW faults too.
>

Great! Then i can expect your patches. ;)

> I just wanted to understand if there was any other reason why you
> needed only wrprotect faults, because the non-present faults didn't
> look like a big performance concern if they triggered in addition to
> wrprotect faults, but it's certainly ok to optimize them away so it's
> fully optimal.
>

Er, you have got the answer, no special, it's only for optimality.

> All it takes to differentiate the behavior should be one more bit
> during registration so you can select non-present, wrprotect faults or
> both. postcopy live migration would select only non-present faults,
> postcopy live snapshot would select only wrprotect faults, anything
> like distributed shared memory supporting shared readonly access and
> exclusive write access, would select both flags.
>

It is really flexible in this way.

> I just sent an (unfortunately) longish but way more detailed email
> about live snapshotting with userfaultfd but I just wanted to give a
> shorter answer here too :).
>

Thanks for your explanation, and your patience. It is really useful,
now i know more details about why 'fork & dump live snapshot' scenario
is not acceptable. Thanks :)

> Thanks,
> Andrea
>
> .
>


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