On Thu, May 18, 2017 at 12:49:41PM +0200, Jan Kara wrote: > Currently, SEEK_HOLE implementation in ext4 may both return that there's > a hole at some offset although that offset already has data and skip > some holes during a search for the next hole. The first problem is > demostrated by: > > xfs_io -c "falloc 0 256k" -c "pwrite 0 56k" -c "seek -h 0" file > wrote 57344/57344 bytes at offset 0 > 56 KiB, 14 ops; 0.0000 sec (2.054 GiB/sec and 538461.5385 ops/sec) > Whence Result > HOLE 0 > > Where we can see that SEEK_HOLE wrongly returned offset 0 as containing > a hole although we have written data there. The second problem can be > demonstrated by: > > xfs_io -c "falloc 0 256k" -c "pwrite 0 56k" -c "pwrite 128k 8k" > -c "seek -h 0" file > > wrote 57344/57344 bytes at offset 0 > 56 KiB, 14 ops; 0.0000 sec (1.978 GiB/sec and 518518.5185 ops/sec) > wrote 8192/8192 bytes at offset 131072 > 8 KiB, 2 ops; 0.0000 sec (2 GiB/sec and 500000.0000 ops/sec) > Whence Result > HOLE 139264 > > Where we can see that hole at offsets 56k..128k has been ignored by the > SEEK_HOLE call. > > The underlying problem is in the ext4_find_unwritten_pgoff() which is > just buggy. In some cases it fails to update returned offset when it > finds a hole (when no pages are found or when the first found page has > higher index than expected), in some cases conditions for detecting hole > are just missing (we fail to detect a situation where indices of > returned pages are not contiguous). > > Fix ext4_find_unwritten_pgoff() to properly detect non-contiguous page > indices and also handle all cases where we got less pages then expected > in one place and handle it properly there. > > CC: stable@xxxxxxxxxxxxxxx > Fixes: c8c0df241cc2719b1262e627f999638411934f60 > CC: Zheng Liu <wenqing.lz@xxxxxxxxxx> > Signed-off-by: Jan Kara <jack@xxxxxxx> Thanks, applied. - Ted
