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

[Hugh Dickins]

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.

(Of course, there's a different thread, in which less reliance on
page lock is being discussed, but that would be a future thing.)

Hugh