Revert "dm: always call blk_queue_split() in dm_process_bio()"

This reverts commit effd58c9.

blk_queue_split() is causing excessive IO splitting -- because
blk_max_size_offset() depends on 'chunk_sectors' limit being set and
if it isn't (as is the case for DM targets!) it falls back to
splitting on a 'max_sectors' boundary regardless of offset.

"Fix" this by reverting back to _not_ using blk_queue_split() in
dm_process_bio() for normal IO (reads and writes).  Long-term fix is
still TBD but it should focus on training blk_max_size_offset() to
call into a DM provided hook (to call DM's max_io_len()).

Test results from simple misaligned IO test on 4-way dm-striped device
with chunksize of 128K and stripesize of 512K:

xfs_io -d -c 'pread -b 2m 224s 4072s' /dev/mapper/stripe_dev

before this revert:

253,0   21        1     0.000000000  2206  Q   R 224 + 4072 [xfs_io]
253,0   21        2     0.000008267  2206  X   R 224 / 480 [xfs_io]
253,0   21        3     0.000010530  2206  X   R 224 / 256 [xfs_io]
253,0   21        4     0.000027022  2206  X   R 480 / 736 [xfs_io]
253,0   21        5     0.000028751  2206  X   R 480 / 512 [xfs_io]
253,0   21        6     0.000033323  2206  X   R 736 / 992 [xfs_io]
253,0   21        7     0.000035130  2206  X   R 736 / 768 [xfs_io]
253,0   21        8     0.000039146  2206  X   R 992 / 1248 [xfs_io]
253,0   21        9     0.000040734  2206  X   R 992 / 1024 [xfs_io]
253,0   21       10     0.000044694  2206  X   R 1248 / 1504 [xfs_io]
253,0   21       11     0.000046422  2206  X   R 1248 / 1280 [xfs_io]
253,0   21       12     0.000050376  2206  X   R 1504 / 1760 [xfs_io]
253,0   21       13     0.000051974  2206  X   R 1504 / 1536 [xfs_io]
253,0   21       14     0.000055881  2206  X   R 1760 / 2016 [xfs_io]
253,0   21       15     0.000057462  2206  X   R 1760 / 1792 [xfs_io]
253,0   21       16     0.000060999  2206  X   R 2016 / 2272 [xfs_io]
253,0   21       17     0.000062489  2206  X   R 2016 / 2048 [xfs_io]
253,0   21       18     0.000066133  2206  X   R 2272 / 2528 [xfs_io]
253,0   21       19     0.000067507  2206  X   R 2272 / 2304 [xfs_io]
253,0   21       20     0.000071136  2206  X   R 2528 / 2784 [xfs_io]
253,0   21       21     0.000072764  2206  X   R 2528 / 2560 [xfs_io]
253,0   21       22     0.000076185  2206  X   R 2784 / 3040 [xfs_io]
253,0   21       23     0.000077486  2206  X   R 2784 / 2816 [xfs_io]
253,0   21       24     0.000080885  2206  X   R 3040 / 3296 [xfs_io]
253,0   21       25     0.000082316  2206  X   R 3040 / 3072 [xfs_io]
253,0   21       26     0.000085788  2206  X   R 3296 / 3552 [xfs_io]
253,0   21       27     0.000087096  2206  X   R 3296 / 3328 [xfs_io]
253,0   21       28     0.000093469  2206  X   R 3552 / 3808 [xfs_io]
253,0   21       29     0.000095186  2206  X   R 3552 / 3584 [xfs_io]
253,0   21       30     0.000099228  2206  X   R 3808 / 4064 [xfs_io]
253,0   21       31     0.000101062  2206  X   R 3808 / 3840 [xfs_io]
253,0   21       32     0.000104956  2206  X   R 4064 / 4096 [xfs_io]
253,0   21       33     0.001138823     0  C   R 4096 + 200 [0]

after this revert:

253,0   18        1     0.000000000  4430  Q   R 224 + 3896 [xfs_io]
253,0   18        2     0.000018359  4430  X   R 224 / 256 [xfs_io]
253,0   18        3     0.000028898  4430  X   R 256 / 512 [xfs_io]
253,0   18        4     0.000033535  4430  X   R 512 / 768 [xfs_io]
253,0   18        5     0.000065684  4430  X   R 768 / 1024 [xfs_io]
253,0   18        6     0.000091695  4430  X   R 1024 / 1280 [xfs_io]
253,0   18        7     0.000098494  4430  X   R 1280 / 1536 [xfs_io]
253,0   18        8     0.000114069  4430  X   R 1536 / 1792 [xfs_io]
253,0   18        9     0.000129483  4430  X   R 1792 / 2048 [xfs_io]
253,0   18       10     0.000136759  4430  X   R 2048 / 2304 [xfs_io]
253,0   18       11     0.000152412  4430  X   R 2304 / 2560 [xfs_io]
253,0   18       12     0.000160758  4430  X   R 2560 / 2816 [xfs_io]
253,0   18       13     0.000183385  4430  X   R 2816 / 3072 [xfs_io]
253,0   18       14     0.000190797  4430  X   R 3072 / 3328 [xfs_io]
253,0   18       15     0.000197667  4430  X   R 3328 / 3584 [xfs_io]
253,0   18       16     0.000218751  4430  X   R 3584 / 3840 [xfs_io]
253,0   18       17     0.000226005  4430  X   R 3840 / 4096 [xfs_io]
253,0   18       18     0.000250404  4430  Q   R 4120 + 176 [xfs_io]
253,0   18       19     0.000847708     0  C   R 4096 + 24 [0]
253,0   18       20     0.000855783     0  C   R 4120 + 176 [0]

Fixes: effd58c9 ("dm: always call blk_queue_split() in dm_process_bio()")
Cc: stable@vger.kernel.org
Reported-by: Andreas Gruenbacher <agruenba@redhat.com>
Tested-by: Barry Marson <bmarson@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm.c

@@ -1739,8 +1739,9 @@ static blk_qc_t dm_process_bio(struct mapped_device *md,
 	 * won't be imposed.
 	 */
 	if (current->bio_list) {
+		if (is_abnormal_io(bio))
 			blk_queue_split(md->queue, &bio);
-		if (!is_abnormal_io(bio))
+		else
 			dm_queue_split(md, ti, &bio);
 	}