Re: EXT3: problem with copy_from_user inside a transaction

[Chris Mason <mason@suse.com>]

On Fri, 2004-09-03 at 08:35, Andrea Arcangeli wrote:
> On Fri, Sep 03, 2004 at 03:05:21PM +0400, Andrey Savochkin wrote:
> > Hi Andrew,
> > 
> > filemap_copy_from_user() between prepare_write() and commit_write()
> > appears to be a problem for ext3.
> > 
And reiserv3, and maybe the other journaled filesystems.

> yes, Chris is working on it for a few months.
> 
Working is a generous term, I've somewhat been waiting for a better
solution to pop into my head.  In the end, I think all we can do is not
allow filesystems to take locks (or implicit locks like starting a
transaction) inside the prepare_write call.

This would mean that all the work is done during the commit_write
stage.  The trick is that we would have to handle -ENOSPC since we might
not know we've run out of room until after the data has been copied from
userland.

prepare_write could reserve blocks, which brings us half way to a
generic delayed allocation layer.  But for a quick and dirty start,
doing it all in commit_write should work.

-chris