On Tue 29-08-17 16:29:42, Andreas Gruenbacher wrote:

From: Christoph Hellwig <hch@lst.de>

Switch to the iomap_seek_hole and iomap_seek_data helpers for
implementing lseek SEEK_HOLE / SEEK_DATA, and remove all the code that
isn't needed any more.

Note that with this patch, ext4 will now always depend on the iomap code
instead of only when CONFIG_DAX is enabled, and it requires adding a
call into the extent status tree for iomap_begin as well to properly
deal with delalloc extents.

Signed-off-by: Christoph Hellwig <hch@lst.de>
[Minor fixes and cleanups by Andreas]
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>

...

One general question about IOMAP_REPORT: When there is unwritten extent, we
use page_cache_seek_hole_data() to determine what is actually a hole and
what is data. We could use exactly the same function to determine what is a
hole and what is data in an IOMAP_HOLE extent, couldn't we? Now I
understand that a filesystem is supposed to return IOMAP_DELALLOC extent if
there is delayed allocation data covering queried offset so probably it's
not a great idea to use that however it still seems a bit like an
unnecessary duplication...

+		if (!ret) {

Please go to this branch only for IOMAP_REPORT case to avoid unnecessary
overhead for DAX mappings...

+			struct extent_status es = {};
+
+			ext4_es_find_delayed_extent_range(inode, map.m_lblk,
+					map.m_lblk + map.m_len - 1, &es);
+			/* Is delalloc data before next block in extent tree? */
+			if (es.es_len && es.es_lblk < map.m_lblk + map.m_len) {

And this is still wrong - I have actually verified that with attached patch
that disables caching of extents (so it is equivalent to *very* aggresive
slab reclaim happening). With that patch applied you'll get:

kvm0:~ # rm /mnt/file; xfs_io -f -c "pwrite 4096 4096" -c "seek -a 0"
/mnt/file
wrote 4096/4096 bytes at offset 4096
4 KiB, 1 ops; 0.0000 sec (16.693 MiB/sec and 4273.5043 ops/sec)
Whence	Result
DATA	0
HOLE	8192

Which is obviously wrong - hole should be 0, data should be 4096.

And the reason why this is wrong is that when we are asked to map things at
first_block and there is a hole at that offset, we need to just truncate
the hole extent returned by ext4_map_blocks() by possibly following
delalloc allocation. But we still need to return that there *is a hole* at
first_block. But your patch seems to try to return the delalloc extent
instead which is wrong.

+				ext4_lblk_t offs = 0;
+
+				if (es.es_lblk < map.m_lblk)
+					offs = map.m_lblk - es.es_lblk;
+				map.m_lblk = es.es_lblk + offs;
+				map.m_pblk = ext4_es_pblock(&es) + offs;
+				map.m_len = es.es_len - offs;
+				if (ext4_es_is_unwritten(&es))
+					map.m_flags |= EXT4_MAP_UNWRITTEN;
+				if (ext4_es_is_delayed(&es))
+					delalloc = true;
+				ret = 1;
+			}
+		}
 	} else {
 		int dio_credits;
 		handle_t *handle;

								Honza
-- 
Jan Kara [off-list ref]
SUSE Labs, CR