To: Andrew Morton <akpm@osdl.org>
Cc: timur.tabi@ammasso.com, hch@infradead.org, hozer@hozed.org,
       linux-kernel@vger.kernel.org, openib-general@openib.org
Subject: Re: [PATCH][RFC][0/4] InfiniBand userspace verbs implementation
References: <200544159.Ahk9l0puXy39U6u6@topspin.com>
	<20050411142213.GC26127@kalmia.hozed.org> <52mzs51g5g.fsf@topspin.com>
	<20050411163342.GE26127@kalmia.hozed.org> <5264yt1cbu.fsf@topspin.com>
	<20050411180107.GF26127@kalmia.hozed.org> <52oeclyyw3.fsf@topspin.com>
	<20050411171347.7e05859f.akpm@osdl.org> <4263DEC5.5080909@ammasso.com>
	<20050418164316.GA27697@infradead.org> <4263E445.8000605@ammasso.com>
	<20050423194421.4f0d6612.akpm@osdl.org> <426BABF4.3050205@ammasso.com>
	<52is2bvvz5.fsf@topspin.com> <20050425135401.65376ce0.akpm@osdl.org>
From: Roland Dreier <roland@topspin.com>
Date: Mon, 25 Apr 2005 14:12:40 -0700
In-Reply-To: <20050425135401.65376ce0.akpm@osdl.org> (Andrew Morton's
 message of "Mon, 25 Apr 2005 13:54:01 -0700")
Message-ID: <521x8yv9vb.fsf@topspin.com>
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    Andrew> Do we care about that?  A straightforward scenario under
    Andrew> which this can happen is:

    Andrew> a) app starts some read I/O in an asynchronous manner
    Andrew> b) app forks
    Andrew> c) child writes to one of the pages which is still under read I/O
    Andrew> d) the read I/O completes
    Andrew> e) the child is left with the old data plus the child's modification instead
    Andrew>    of the new data

    Andrew> which is a very silly application which is giving itself
    Andrew> unpredictable memory contents anyway.

    Andrew> I assume there's a more sensible scenario?

You're right, that is a silly scenario ;)  In fact if we mark vmas
with VM_DONTCOPY, then the child just crashes with a seg fault.

The type of thing I'm worried about is something like, for example:

a) app registers memory region with RDMA hardware -- in other words,
   loads the device's translation table for future I/O
b) app forks
c) app writes to the registered memory region, and the kernel breaks
   the COW for the (now read-only) page by mapping a new page
d) app starts an I/O that will do a DMA read from the region
e) device reads using the wrong, old mapping

This can be pretty insiduous because for example fork() + immediate
exec() or just using system() still leaves the parent with PTEs marked
read-only.  If an application does overlapping memory registrations so
get_user_pages() is called a lot, then as far as I can see
can_share_swap_page() will always return 0 and the COW will happen
even if the child process has thrown out its original vmas.

Or if the counts are in the correct range, then there's a small window
between fork() and exec() where the parent process can screw itself
up, so most of the time the app works, until it doesn't.

 - R.
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