linux-kernel - Re: [PATCH 14/23] userfaultfd: wake pending userfaults

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Message-ID: <20151022151509.GO3604@twins.programming.kicks-ass.net>
Date:	Thu, 22 Oct 2015 17:15:09 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	qemu-devel@...gnu.org, kvm@...r.kernel.org,
	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 14/23] userfaultfd: wake pending userfaults

On Thu, Oct 22, 2015 at 04:18:31PM +0200, Andrea Arcangeli wrote:

> The risk of memory corruption is still zero no matter what happens
> here, in the extremely rare case the app will get a SIGBUS or a

That might still upset people, SIGBUS isn't something an app can really
recover from.

> I'm not exactly sure why we allow VM_FAULT_RETRY only once currently
> so I'm tempted to drop FAULT_FLAG_TRIED entirely.

I think to ensure we make forward progress.

> I've no real preference on how to tweak the page fault code to be able
> to return VM_FAULT_RETRY indefinitely and I would aim for the smallest
> change possible, so if you've suggestions now it's good time.

Indefinitely is such a long time, we should try and finish
computation before the computer dies etc. :-)

Yes, yes.. I know, extremely unlikely etc. Still guarantees are good.


In any case, I'm not really too bothered how you fix it, just figured
I'd let you know.
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