Re: [PATCH 1/2] dax: prevent invalidation of mapped DAX entries

[Ross Zwisler <ross.zwisler-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>]

On Tue, Apr 25, 2017 at 12:10:41PM +0200, Jan Kara wrote:
> On Thu 20-04-17 21:44:36, Ross Zwisler wrote:
> > dax_invalidate_mapping_entry() currently removes DAX exceptional entries
> > only if they are clean and unlocked.  This is done via:
> > 
> > invalidate_mapping_pages()
> >   invalidate_exceptional_entry()
> >     dax_invalidate_mapping_entry()
> > 
> > However, for page cache pages removed in invalidate_mapping_pages() there
> > is an additional criteria which is that the page must not be mapped.  This
> > is noted in the comments above invalidate_mapping_pages() and is checked in
> > invalidate_inode_page().
> > 
> > For DAX entries this means that we can can end up in a situation where a
> > DAX exceptional entry, either a huge zero page or a regular DAX entry,
> > could end up mapped but without an associated radix tree entry. This is
> > inconsistent with the rest of the DAX code and with what happens in the
> > page cache case.
> > 
> > We aren't able to unmap the DAX exceptional entry because according to its
> > comments invalidate_mapping_pages() isn't allowed to block, and
> > unmap_mapping_range() takes a write lock on the mapping->i_mmap_rwsem.
> > 
> > Since we essentially never have unmapped DAX entries to evict from the
> > radix tree, just remove dax_invalidate_mapping_entry().
> > 
> > Signed-off-by: Ross Zwisler <ross.zwisler-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>
> > Fixes: c6dcf52c23d2 ("mm: Invalidate DAX radix tree entries only if appropriate")
> > Reported-by: Jan Kara <jack-AlSwsSmVLrQ@public.gmane.org>
> > Cc: <stable-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>    [4.10+]
> 
> Just as a side note - we wouldn't really have to unmap the mapping range
> covered by the DAX exceptional entry. It would be enough to find out
> whether such range is mapped and bail out in that case. But that would
> still be pretty expensive for DAX - we'd have to do rmap walk similar as in
> dax_mapping_entry_mkclean() and IMHO it is not worth it. So I agree with
> what you did. You can add:
> 
> Reviewed-by: Jan Kara <jack-AlSwsSmVLrQ@public.gmane.org>

Yep, that makes sense.  Thanks for the review.