Re: hole-punch vs fault

[Jan Kara <jack@suse.cz>]

On Mon 02-12-13 08:58:25, Matthew Wilcox wrote:
> On Mon, Dec 02, 2013 at 09:33:18AM +0100, Jan Kara wrote:
> > > Indeed, XIP could make use of the structures we already
> > > have in the struct inode/address space to behave more like a normal
> > > filesystem. We already use the mapping tree to store
> > > per-page information that is used to serialise truncate vs page
> > > faults, so why not make XIP do exactly the same thing? 
> >   I believe that grabbing mmap_sem for writing during truncate (in case of
> > ext2 around xip_truncate_page() & truncate_setsize() calls should do the
> > trick. But I need to verify with lockdep that it doesn't introduce new
> > locking problems.
> 
> Umm ... mmap_sem for write?  On each of the tasks that have a file mmaped?
> I know we have double_down that orders by memory address, but I don't
> think we have an N_down_write().
  No, sorry, that was my mistake. I was thinking about threads of a single
process racing but the race is there obviously for different processes as
well so mmap_sem won't help us.

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