Re: Uninitialized extent races

[Zheng Liu <gnehzuil.liu@gmail.com>]

On Mon, Dec 31, 2012 at 09:32:21AM +0100, Jan Kara wrote:
> On Mon 24-12-12 19:17:45, Zheng Liu wrote:
> > On Fri, Dec 21, 2012 at 01:02:43PM -0500, Theodore Ts'o wrote:
> > > On Fri, Dec 21, 2012 at 05:19:29PM +0100, Jan Kara wrote:
> > > >   No, I'm speaking about merging currently uninitialized extents. I.e.
> > > > suppose someone does the following on a filesystem with dioread_nolock so
> > > > that writeback happens via unwritten extents:
> > > >   fd = open("file", O_RDWR);
> > > >   pwrite(fd, buf, 4096, 0);
> > > > 					flusher thread starts writing
> > > > 					we create uninitialized extent for
> > > > 					  range 0-4096
> > > >   fallocate(fd, 0, 4096, 4096);
> > > >     - we merge extents and now have just 1 uninitialized extent for range
> > > >       0-8192
> > > > 					ext4_convert_unwritten_extents() now
> > > > 					  has to split the extent to finish
> > > > 					  the IO.
> > > 
> > > Ah, I see.  Disabling the the merging that might take place as a
> > > result of the fallocate.  Yes, I agree that's a completely sane thing
> > > to do.
> > > 
> > > The alternate approach would be to add a flag in the extent status
> > > tree indicating that an unwritten conversion is pending, but that
> > > would add more complexity.
> > 
> > Sorry for delay reply.  Indeed we could add a flag in extent status tree
> > to indicate an pending unwritten extent, and I believe that it can bring
> > us some benefits.  But I wonder whether this case often happens.  Do we
> > have some real workloads?
>   It doesn't happen often but it *can* happen. Thus you have to implement
> a code which handles the case. I don't think bit in extent status tree is
> really necessary. Just disabling merging of uninitialized extents is
> simple. If we see there are some real workloads which have problems with
> it, we can resort to a more complex solution using extent tree...

Thanks for your explanation.  I don't know whether or not you have
generated a patch for this problem.  I am willing to make it in a proper
time.  If you have begun to generate it, please let me know. :-)

Happy New Year,
                                                - Zheng