xfs: Introduce writeback context for writepages
xfs_vm_writepages() calls generic_writepages to writeback a range of a file, but then xfs_vm_writepage() clusters pages itself as it does not have any context it can pass between->writepage calls from __write_cache_pages(). Introduce a writeback context for xfs_vm_writepages() and call __write_cache_pages directly with our own writepage callback so that we can pass that context to each writepage invocation. This encapsulates the current mapping, whether it is valid or not, the current ioend and it's IO type and the ioend chain being built. This requires us to move the ioend submission up to the level where the writepage context is declared. This does mean we do not submit IO until we packaged the entire writeback range, but with the block plugging in the writepages call this is the way IO is submitted, anyway. It also means that we need to handle discontiguous page ranges. If the pages sent down by write_cache_pages to the writepage callback are discontiguous, we need to detect this and put each discontiguous page range into individual ioends. This is needed to ensure that the ioend accurately represents the range of the file that it covers so that file size updates during IO completion set the size correctly. Failure to take into account the discontiguous ranges results in files being too small when writeback patterns are non-sequential. Signed-off-by: NDave Chinner <dchinner@redhat.com> Reviewed-by: NChristoph Hellwig <hch@lst.de> Signed-off-by: NDave Chinner <david@fromorbit.com>
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