The flush in xfs_buffered_write_iomap_begin() for zero range over a data fork hole fronted by COW fork prealloc is primarily designed to provide correct zeroing behavior in particular pagecache conditions. As it turns out, this also partially masks some odd behavior in insert range (via zero range via setattr). Insert range bumps i_size the length of the new range, flushes, unmaps pagecache and cancels COW prealloc, and then right shifts extents from the end of the file back to the target offset of the insert. Since the i_size update occurs before the pagecache flush, this creates a transient situation where writeback around EOF can behave differently. This appears to be corner case situation, but if happens to be fronted by COW fork speculative preallocation and a large, dirty folio that contains at least one full COW block beyond EOF, the writeback after i_size is bumped may remap that COW fork block into the data fork within EOF. The block is zeroed and then shifted back out to post-eof, but this is unexpected in that it leads to a written post-eof data fork block. This can cause a zero range warning on a subsequent size extension, because we should never find blocks that require physical zeroing beyond i_size. To avoid this quirk, flush the EOF folio before the i_size update during insert range. The entire range will be flushed, unmapped and invalidated anyways, so this should be relatively unnoticeable. Signed-off-by: Brian Foster --- fs/xfs/xfs_file.c | 17 +++++++++++++++++ 1 file changed, 17 insertions(+) diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c index 7874cf745af3..1f2730558165 100644 --- a/fs/xfs/xfs_file.c +++ b/fs/xfs/xfs_file.c @@ -1227,6 +1227,23 @@ xfs_falloc_insert_range( if (offset >= isize) return -EINVAL; + /* + * Let writeback clean up EOF folio state before we bump i_size. The + * insert flushes before it starts shifting and under certain + * circumstances we can write back blocks that should technically be + * considered post-eof (and thus should not be submitted for writeback). + * + * For example, a large, dirty folio that spans EOF and is backed by + * post-eof COW fork preallocation can cause block remap into the data + * fork. This shifts back out beyond EOF, but creates an expectedly + * written post-eof block. The insert is going to flush, unmap and + * cancel prealloc across this whole range, so flush EOF now before we + * bump i_size to provide consistent behavior. + */ + error = filemap_write_and_wait_range(inode->i_mapping, isize, isize); + if (error) + return error; + error = xfs_falloc_setsize(file, isize + len); if (error) return error; -- 2.52.0