Re: [PATCH, RFC] fix null files exposure growing via ftruncate

[David Chinner <dgc@sgi.com>]

On Thu, Jun 14, 2007 at 07:14:46PM +0100, Christoph Hellwig wrote:
> On Thu, Jun 14, 2007 at 04:34:04PM +1000, David Chinner wrote:
> > Christoph,
> > 
> > Looking into the test 140 failure you reported, I realised that none
> > of the specific null files tests were being run automatically, which
> > is why I hadn't seen any of those failures (nor had the QA team).
> > That's being fixed.
> > 
> > I suspect that the test passes on Irix because of a coincidence
> > (the test sleeps for 10s and that is the default writeback
> > timeout for file data) which means when the filesystem is shut down
> > all the data is already on disk so it's not really testing
> > the NULL files fix.
> > 
> > The failure is due to the ftruncate() logging the new file size
> > before any data that had previously been written had hit the
> > disk. IOWs, it violates the data write/inode size update rule
> > that fixes the null files problem.
> > 
> > The fix here checks when growing the file as to whether it the disk
> > inode size is different to the in memory size. If they are
> > different, we have data that needs to be written to disk beyond the
> > existing on disk EOF. Hence to maintain ordering we need to flush
> > this data out before we log the changed file size.
> > 
> > I suspect the flush could be done more optimally - I've just done a
> > brute-force flush the entire file mod. Should we only flush from the
> > old di_size to the current i_size?
> > 
> > There may also be better ways to fix this. Any thoughts on
> > that?
> 
> Looks good enough for now, but I suspect just flushing from the old
> to the new size would be a quite nice performance improvement that's
> worth it.

Yeah, that's the way I was leaning. I'll mod the patch to do that and
repost. Thanks.

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group