Re: More filesystem need this fix (xfs: use MMAPLOCK around filemap_map_pages())

[Dave Chinner <david@fromorbit.com>]

On Wed, Sep 16, 2020 at 07:04:46PM -0700, Hugh Dickins wrote:
> On Thu, 17 Sep 2020, Dave Chinner wrote:
> > 
> > So....
> > 
> > P0					p1
> > 
> > hole punch starts
> >   takes XFS_MMAPLOCK_EXCL
> >   truncate_pagecache_range()
> >     unmap_mapping_range(start, end)
> >       <clears ptes>
> > 					<read fault>
> > 					do_fault_around()
> > 					  ->map_pages
> > 					    filemap_map_pages()
> > 					      page mapping valid,
> > 					      page is up to date
> > 					      maps PTEs
> > 					<fault done>
> >     truncate_inode_pages_range()
> >       truncate_cleanup_page(page)
> >         invalidates page
> >       delete_from_page_cache_batch(page)
> >         frees page
> > 					<pte now points to a freed page>
> 
> No.  filemap_map_pages() checks page->mapping after trylock_page(),
> before setting up the pte; and truncate_cleanup_page() does a one-page
> unmap_mapping_range() if page_mapped(), while holding page lock.

Ok, fair, I missed that.

So why does truncate_pagecache() talk about fault races and require
a second unmap range after the invalidation "for correctness" if
this sort of race cannot happen?

Why is that different to truncate_pagecache_range() which -doesn't-i
do that second removal? It's called for more than just hole_punch -
from the filesystem's persepective holepunch should do exactly the
same as truncate to the page cache, and for things like
COLLAPSE_RANGE it is absolutely essential because the data in that
range is -not zero- and will be stale if the mappings are not
invalidated completely....

Also, if page->mapping == NULL is sufficient to detect an invalidated
page in all cases, then why does page_cache_delete() explicitly
leave page->index intact:

	page->mapping = NULL;
	/* Leave page->index set: truncation lookup relies upon it */


Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com