Re: [PATCH] Fix ext2 error reporting on fsync

[Jan Kara <jack@suse.cz>]

> Chris Mason <mason@suse.com> wrote:
> >
> > On Thursday 12 January 2006 15:47, Andrew Morton wrote:
> > > Jan Kara <jack@suse.cz> wrote:
> > > > > Jan Kara <jack@suse.cz> wrote:
> > > > >
> > > > > hm, yes, try_to_free_buffers() against a blockdev page is a problem. 
> > > > > But I think it's fixable.
> > > > >
> > > > > In the second loop of drop_buffers(), if we find a bh which had a write
> > > > > error we need to set AS_EIO on the address_space which is interested in
> > > > > this buffer.
> > > > >
> > > > > That address_space is pinned by a) the fact that it has buffers at
> > > > > ->private_list and b) we hold the blockdev mapping's private_lock.
> > > > >
> > > > > The only problem is actually _finding_ the address_space which is
> > > > > interested in this buffer_head.  Looks like we'd need a backpointer in
> > > > > the buffer_head.
> > > >
> > > >   Yes, the pointer seems to be inevitable in this solution. If you think
> > > > the improvement is worth adding 4 bytes to each buffer_head, then I can
> > > > write the patch.
> > >
> > > hm.  It only affects what are now rarely-used filesystems like ext2, minix,
> > > etc.  Not sure about reiser3.  But it is a strict correctness issue.  I
> > > suppose we should do it.  It'd be nice to find a way to avoid increasing
> > > the bh size..
> > >
> > 
> > I know my patch is pretty nasty, but is it really worth adding complexity to 
> > the base code for this corner case?
> > 
> 
> It's not much complexity: a few set_bits and test_and_clear_bits in three
> places.  The main drawback is a larger buffer_head.
> 
> Yeah, it's a pita, but it is a data-integrity correctness thing.
  There's a way to avoid the extra pointer in buffer_head: we could
change the circular linked list b_assoc_buffers to a non-circular one
(probably use hlists). Then if we find a buffer with IO error, we could
find a list head and from it compute the pointer to the mapping... It
would not be fast but on IO error I think we could afford it. Do you think
it's a good idea?

									Honza
-- 
Jan Kara <jack@suse.cz>
SuSE CR Labs