linux-kernel - Re: [PATCH 2/3] fadvise: Add _VOLATILE,_ISVOLATILE, and

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Message-ID: <20120501000815.GQ7015@dastard>
Date:	Tue, 1 May 2012 10:08:15 +1000
From:	Dave Chinner <david@...morbit.com>
To:	John Stultz <john.stultz@...aro.org>
Cc:	Dave Hansen <dave@...ux.vnet.ibm.com>,
	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>,
	Rik van Riel <riel@...hat.com>,
	Dmitry Adamushko <dmitry.adamushko@...il.com>,
	Neil Brown <neilb@...e.de>,
	Andrea Righi <andrea@...terlinux.com>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Subject: Re: [PATCH 2/3] fadvise: Add _VOLATILE,_ISVOLATILE, and _NONVOLATILE
 flags

On Mon, Apr 30, 2012 at 02:07:16PM -0700, John Stultz wrote:
> On 04/27/2012 06:36 PM, Dave Chinner wrote:
> >That's my concern - that persistent filesystems will have different
> >behaviour to in-memory filesystems. They *must* be consistent in
> >behaviour w.r.t. to stale data exposure, otherwise we are in a world
> >of pain when applications start to use this. Quite frankly, I don't
> >care about performance of VOLATILE ranges, but I care greatly
> >about ensuring filesystems don't expose stale data to user
> >applications....
> >
> I think we're in agreement with the rest of this email, but I do
> want to stress that the performance of volatile ranges will become
> more ciritical, as in order for folks to effectively use them, they
> need to be able to mark and unmark ranges any time they're not using
> the data.

Performance is far less important than data security. Make it safe
first, then optimise performance. As it is, the initial target of
tmpfs - by it's very nature of returning zeros for regions not
backed by pages - is safe w.r.t. stale data exposure, so it will not
be slowed down by using an fallocate "best effort" hole-punching
interface.  The performance of other filesystems is something that
the relevant filesystem developers can worry about....

> So if the overhead is too great for marking and unmarking pages,
> applications will be less likely to "help out".  :)

Devil's Advocate: If the benefit of managing caches in such a manner
is this marginal, then why add the complexity to the kernel?

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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