linux-kernel - Re: [PATCH 0/2] [RFC] Volatile Ranges (v6)

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Date:	Sat, 7 Apr 2012 10:14:13 +0200
From:	Dmitry Adamushko <dmitry.adamushko@...il.com>
To:	John Stultz <john.stultz@...aro.org>
Cc:	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@...ux.vnet.ibm.com>,
	Rik van Riel <riel@...hat.com>,
	Dave Chinner <david@...morbit.com>, Neil Brown <neilb@...e.de>,
	Andrea Righi <andrea@...terlinux.com>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Subject: Re: [PATCH 0/2] [RFC] Volatile Ranges (v6)

On 7 April 2012 02:08, John Stultz <john.stultz@...aro.org> wrote:
>
> Another detail is that by hanging the volatile ranges off of the
> address_space, the volatility for tmpfs files persists even when no one
> has an open fd on the file. This could cause some surprises if application
> A marked some pages volatile and died, then application B opened the file
> and had pages dropped out underneith it while it was being used. I suspect
> I need to clean up the volatility when all fds are dropped.

And how do you handle the regions that have already been purged by
this moment? Unless B has some specific mechanism to verify the
consistency of the content, a sensible way would be to always mark off
the regions as non-volatile before accessing them and verify the
return code to see if there are holes.

More generally, what if B opens the file while A is still working with
it? Besides the use of normal synchronization mechanisms, B should not
make any assumption on the current state of the regions (unless there
is a high-level protocol between A and B to share this info). So an
explicit mark-off-as-non_volatile could be a simple generic mechanism.

--Dmitry
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