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Re: [dm-devel] [Lsf] Preliminary Agenda and Activities for LSF



On Fri, Apr 1, 2011 at 2:46 PM, Joel Becker <jlbec evilplan org> wrote:
> On Fri, Apr 01, 2011 at 09:30:04AM -0700, Amir Goldstein wrote:
>> when writing DIO to indirect mapped file holes, we fall back to buffered write
>> (so we won't expose stale data in the case of a crash) concurrent DIO reads
>> to that file (before data writeback) can expose stale data. right?
>> do you consider this case mixing buffered and DIO access?
>> do you consider that as a problem?
>
>        I do not consider this 'mixing', nor do I consider it a problem.
> ocfs2 does exactly this for holes, unwritten extents, and CoW.  It does
> not violate the user's expectation that the data will be on disk when
> the write(2) returns.
>        Falling back to buffered on read(2) is a different story; the
> caller wants the current state of the disk block, not five minutes ago.
> So we can't do that.  But we also don't need to.

the issue is with DIO read exposing uninitialized data on disk
is a security issue.
it's not about giving the read what is expects to see.

>        O_DIRECT users that are worried about any possible space usage in
> the page cache have already pre-allocated their disk blocks and don't
> get here.
>
> Joel
>
> --
>
> "Under capitalism, man exploits man.  Under Communism, it's just
>   the opposite."
>                                 - John Kenneth Galbraith
>
>                        http://www.jlbec.org/
>                        jlbec evilplan org
>


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