Re: [PATCH 3/5 resend] VFS: Fix s_umount thaw/write deadlock

[Jan Kara <jack@suse.cz>]

On Wed 07-12-11 16:49:20, Matthew Wilcox wrote:
> On Thu, Dec 08, 2011 at 12:16:58AM +0100, Jan Kara wrote:
> > On Tue 06-12-11 06:35:44, Christoph Hellwig wrote:
> > > We make sure to not dirty any new inodes after the first phase of the
> > > freeze, so this should be a BUG_ON/WARN_ON.
> >   This is not really true in presence of mmaped writes. To block mmaped
> > writes on a frozen filesystem, we need some synchronization between
> > page_mkwrite() and freezing code. Currently, to avoid any additional
> > locking overhead, we set page dirty and *then* check for filesystem being
> > frozen. Only this order can make sure either the page is written (and
> > write-protected) or the frozen check triggers and we wait... (see the
> > comment in block_page_mkwrite()). The nasty sideeffect of this is that
> > there can be dirty pages & inodes on a frozen filesystem. We are blocked in
> > the page fault of these pages so user cannot write any data to these pages
> > but still they are marked dirty.
> 
> Can't we make the mmaps read-only during the freezing process?
  But then application will get EFAULT instead of blocking? I don't think
that is acceptable (we'd see lots of applications dying ;).

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