linux-kernel - Re: More filesystem need this fix (xfs: use MMAPLOCK around filemap_map

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Date:   Wed, 16 Sep 2020 19:04:46 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     Dave Chinner <david@...morbit.com>
cc:     Jan Kara <jack@...e.cz>, Amir Goldstein <amir73il@...il.com>,
        Andreas Gruenbacher <agruenba@...hat.com>,
        Theodore Tso <tytso@....edu>,
        Martin Brandenburg <martin@...ibond.com>,
        Mike Marshall <hubcap@...ibond.com>,
        Damien Le Moal <damien.lemoal@....com>,
        Jaegeuk Kim <jaegeuk@...nel.org>,
        Qiuyang Sun <sunqiuyang@...wei.com>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Matthew Wilcox <willy@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>, nborisov@...e.de
Subject: Re: More filesystem need this fix (xfs: use MMAPLOCK around
 filemap_map_pages())

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