linux-kernel - Re: [PATCH 0/5] Volatile Ranges (v12) & LSF-MM discussion fodder

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Date:	Tue, 01 Apr 2014 21:07:02 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	John Stultz <john.stultz@...aro.org>,
	Johannes Weiner <hannes@...xchg.org>
CC:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Android Kernel Team <kernel-team@...roid.com>,
	Robert Love <rlove@...gle.com>, Mel Gorman <mel@....ul.ie>,
	Hugh Dickins <hughd@...gle.com>, Dave Hansen <dave@...1.net>,
	Rik van Riel <riel@...hat.com>,
	Dmitry Adamushko <dmitry.adamushko@...il.com>,
	Neil Brown <neilb@...e.de>,
	Andrea Arcangeli <aarcange@...hat.com>,
	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>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH 0/5] Volatile Ranges (v12) & LSF-MM discussion fodder

On 04/01/2014 09:03 PM, John Stultz wrote:
> 
> So, maybe its best to ignore the fact that folks want to do semi-crazy
> user-space faulting via SIGBUS. At least to start with. Lets look at the
> semantic for the "normal" mark volatile, never touch the pages until you
> mark non-volatile - basically where accessing volatile pages is similar
> to a use-after-free bug.
> 
> So, for the most part, I'd say the proposed SIGBUS semantics don't
> complicate things for this basic use-case, at least when compared with
> things like zero-fill.  If an applications accidentally accessed a
> purged volatile page, I think SIGBUS is the right thing to do. They most
> likely immediately crash, but its better then them moving along with
> silent corruption because they're mucking with zero-filled pages.
> 
> So between zero-fill and SIGBUS, I think SIGBUS makes the most sense. If
> you have a third option you're thinking of, I'd of course be interested
> in hearing it.
> 

People already do SIGBUS for mmap, so there is nothing new here.

> Now... once you've chosen SIGBUS semantics, there will be folks who will
> try to exploit the fact that we get SIGBUS on purged page access (at
> least on the user-space side) and will try to access pages that are
> volatile until they are purged and try to then handle the SIGBUS to fix
> things up. Those folks exploiting that will have to be particularly
> careful not to pass volatile data to the kernel, and if they do they'll
> have to be smart enough to handle the EFAULT, etc. That's really all
> their problem, because they're being clever. :)

Yep.

	-hpa

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