linux-kernel - Re: [PATCH 00/23] userfaultfd v4

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Message-ID: <CAFLxGvwGGZH1bbMw+qReZFMK+dc6zoOTCNsuOMdp+xw_jPzPDg@mail.gmail.com>
Date:	Tue, 19 May 2015 23:59:42 +0200
From:	Richard Weinberger <richard.weinberger@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Andrea Arcangeli <aarcange@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>, qemu-devel@...gnu.org,
	kvm <kvm@...r.kernel.org>,
	"open list:ABI/API" <linux-api@...r.kernel.org>,
	Pavel Emelyanov <xemul@...allels.com>,
	Sanidhya Kashyap <sanidhya.gatech@...il.com>,
	zhang.zhanghailiang@...wei.com,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Andres Lagar-Cavilla <andreslc@...gle.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
	Andy Lutomirski <luto@...capital.net>,
	Hugh Dickins <hughd@...gle.com>,
	Peter Feiner <pfeiner@...gle.com>,
	"Dr. David Alan Gilbert" <dgilbert@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	"Huangpeng (Peter)" <peter.huangpeng@...wei.com>
Subject: Re: [PATCH 00/23] userfaultfd v4

On Tue, May 19, 2015 at 11:38 PM, Andrew Morton
<akpm@...ux-foundation.org> wrote:
> On Thu, 14 May 2015 19:30:57 +0200 Andrea Arcangeli <aarcange@...hat.com> wrote:
>
>> This is the latest userfaultfd patchset against mm-v4.1-rc3
>> 2015-05-14-10:04.
>
> It would be useful to have some userfaultfd testcases in
> tools/testing/selftests/.  Partly as an aid to arch maintainers when
> enabling this.  And also as a standalone thing to give people a
> practical way of exercising this interface.
>
> What are your thoughts on enabling userfaultfd for other architectures,
> btw?  Are there good use cases, are people working on it, etc?

UML is using SIGSEGV for page faults.
i.e. the UML processes receives a SIGSEGV, learns the faulting address
from the mcontext
and resolves the fault by installing a new mapping.

If userfaultfd is faster that the SIGSEGV notification it could speed
up UML a bit.
For UML I'm only interested in the notification, not the resolving
part. The "missing"
data is present, only a new mapping is needed. No copy of data.

Andrea, what do you think?

-- 
Thanks,
//richard
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