linux-kernel - Re: [PATCH 0/6] enable writing to /proc/pid/mem

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Message-ID: <20110309023303.GZ22723@ZenIV.linux.org.uk>
Date:	Wed, 9 Mar 2011 02:33:04 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Stephen Wilson <wilsons@...rt.ca>
Cc:	linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Roland McGrath <roland@...hat.com>,
	Matt Mackall <mpm@...enic.com>,
	David Rientjes <rientjes@...gle.com>,
	Nick Piggin <npiggin@...nel.dk>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Mel Gorman <mel@....ul.ie>, Ingo Molnar <mingo@...e.hu>,
	Michel Lespinasse <walken@...gle.com>,
	Hugh Dickins <hughd@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/6] enable writing to /proc/pid/mem

On Tue, Mar 08, 2011 at 09:15:25PM -0500, Stephen Wilson wrote:

> I think we could also remove the intermediate copy in both mem_read() and
> mem_write() as well, but I think such optimizations could be left for
> follow on patches.

How?  We do copy_.._user() in there; it can trigger page faults and
that's not something you want while holding mmap_sem on some mm.
Looks like a deadlock country...  So we can't do that from inside
access_process_vm() or its analogs, which means buffering in caller.
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