Subject: Re: ext3_ordered_writepage() questions
From: Badari Pulavarty <pbadari@us.ibm.com>
To: "Theodore Ts'o" <tytso@mit.edu>
Cc: Andrew Morton <akpm@osdl.org>, sct@redhat.com,
       lkml <linux-kernel@vger.kernel.org>,
       linux-fsdevel <linux-fsdevel@vger.kernel.org>, jack@suse.cz
In-Reply-To: <20060316180904.GA29275@thunk.org>
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Date: Thu, 16 Mar 2006 10:22:40 -0800
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On Thu, 2006-03-16 at 13:09 -0500, Theodore Ts'o wrote:
> > >Yup.  Ordered-mode JBD commit needs to write and wait upon all dirty
> > >file-data buffers prior to journalling the metadata.  If we didn't run
> > >journal_dirty_data_fn() against those buffers then they'd still be under
> > >I/O after commit had completed.
> > >
> > In  non-block allocation case, what metadata are we journaling in 
> > writepage() ?
> > block allocation happend in prepare_write() and commit_write() journaled the
> > transaction. All the meta data updates should be done there.  What JBD 
> > commit are you refering to here ?
> 
> Basically, this boils down to what is our definition of ordered-mode?
> 
> If the goal is to make sure we avoid the security exposure of
> allocating a block and then crashing before we write the data block,
> potentially exposing previously written data that might be belong to
> another user, then what Badari is suggesting would avoid this
> particular problem.

Yes, if the block allocation is needed, my patch is basically
no-op, we go through regular code.

> However, if what we are doing is overwriting our own data with more an
> updated, more recent version of the data block, do we guarantee that
> any ordering semantics apply?  For example, what if we write a data
> block, and then follow it up with some kind of metadata update (say we
> touch atime, or add an extended attribute).  Do we guarantee that if
> the metadata update is committed, that the data block will have made
> it to disk as well?  

I don't see how we do this today. Yes. Metadata updates are jounalled,
but I don't see how current adding buffers through journal_dirty_data
(bh) call can guarantee that these buffers get added to metadata-update
transaction ?


> Today that is the way things work, but is that
> guarantee part of the contract of ordered-mode?


BTW, thanks Ted for putting this in human-readable terms :)

Thanks,
Badari

						
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