Re: [PATCH 2/3] fadvise: Add _VOLATILE,_ISVOLATILE, and _NONVOLATILE flags

[Dave Chinner <david@fromorbit.com>]

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@fromorbit.com