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xfs: trim writepage mapping to within eof
The writeback rework in commit fbcc0256 ("xfs: Introduce writeback context for writepages") introduced a subtle change in behavior with regard to the block mapping used across the ->writepages() sequence. The previous xfs_cluster_write() code would only flush pages up to EOF at the time of the writepage, thus ensuring that any pages due to file-extending writes would be handled on a separate cycle and with a new, updated block mapping. The updated code establishes a block mapping in xfs_writepage_map() that could extend beyond EOF if the file has post-eof preallocation. Because we now use the generic writeback infrastructure and pass the cached mapping to each writepage call, there is no implicit EOF limit in place. If eofblocks trimming occurs during ->writepages(), any post-eof portion of the cached mapping becomes invalid. The eofblocks code has no means to serialize against writeback because there are no pages associated with post-eof blocks. Therefore if an eofblocks trim occurs and is followed by a file-extending buffered write, not only has the mapping become invalid, but we could end up writing a page to disk based on the invalid mapping. Consider the following sequence of events: - A buffered write creates a delalloc extent and post-eof speculative preallocation. - Writeback starts and on the first writepage cycle, the delalloc extent is converted to real blocks (including the post-eof blocks) and the mapping is cached. - The file is closed and xfs_release() trims post-eof blocks. The cached writeback mapping is now invalid. - Another buffered write appends the file with a delalloc extent. - The concurrent writeback cycle picks up the just written page because the writeback range end is LLONG_MAX. xfs_writepage_map() attributes it to the (now invalid) cached mapping and writes the data to an incorrect location on disk (and where the file offset is still backed by a delalloc extent). This problem is reproduced by xfstests test generic/464, which triggers racing writes, appends, open/closes and writeback requests. To address this problem, trim the mapping used during writeback to within EOF when the mapping is validated. This ensures the mapping is revalidated for any pages encountered beyond EOF as of the time the current mapping was cached or last validated. Reported-by:Eryu Guan <eguan@redhat.com> Diagnosed-by: