[PATCH] Revert "block: simplify set_init_blocksize" to regain lost performance

[Maxim Mikityanskiy]

The cited commit introduced a serious regression with SATA write speed,
as found by bisecting. This patch reverts this commit, which restores
write speed back to the values observed before this commit.

The performance tests were done on a Helios4 NAS (2nd batch) with 4 HDDs
(WD8003FFBX) using dd (bs=1M count=2000). "Direct" is a test with a
single HDD, the rest are different RAID levels built over the first
partitions of 4 HDDs. Test results are in MB/s, R is read, W is write.

                | Direct | RAID0 | RAID10 f2 | RAID10 n2 | RAID6
----------------+--------+-------+-----------+-----------+--------
9011495c9466    | R:256  | R:313 | R:276     | R:313     | R:323
(before faulty) | W:254  | W:253 | W:195     | W:204     | W:117
----------------+--------+-------+-----------+-----------+--------
5ff9f19231a0    | R:257  | R:398 | R:312     | R:344     | R:391
(faulty commit) | W:154  | W:122 | W:67.7    | W:66.6    | W:67.2
----------------+--------+-------+-----------+-----------+--------
5.10.10         | R:256  | R:401 | R:312     | R:356     | R:375
unpatched       | W:149  | W:123 | W:64      | W:64.1    | W:61.5
----------------+--------+-------+-----------+-----------+--------
5.10.10         | R:255  | R:396 | R:312     | R:340     | R:393
patched         | W:247  | W:274 | W:220     | W:225     | W:121

Applying this patch doesn't hurt read performance, while improves the
write speed by 1.5x - 3.5x (more impact on RAID tests). The write speed
is restored back to the state before the faulty commit, and even a bit
higher in RAID tests (which aren't HDD-bound on this device) - that is
likely related to other optimizations done between the faulty commit and
5.10.10 which also improved the read speed.

Signed-off-by: Maxim Mikityanskiy <maxtram95@xxxxxxxxx>
Fixes: 5ff9f19231a0 ("block: simplify set_init_blocksize")
Cc: Christoph Hellwig <hch@xxxxxx>
Cc: Jens Axboe <axboe@xxxxxxxxx>
---
 fs/block_dev.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/fs/block_dev.c b/fs/block_dev.c
index 3b8963e228a1..235b5042672e 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -130,7 +130,15 @@ EXPORT_SYMBOL(truncate_bdev_range);
 
 static void set_init_blocksize(struct block_device *bdev)
 {
-	bdev->bd_inode->i_blkbits = blksize_bits(bdev_logical_block_size(bdev));
+	unsigned int bsize = bdev_logical_block_size(bdev);
+	loff_t size = i_size_read(bdev->bd_inode);
+
+	while (bsize < PAGE_SIZE) {
+		if (size & bsize)
+			break;
+		bsize <<= 1;
+	}
+	bdev->bd_inode->i_blkbits = blksize_bits(bsize);
 }
 
 int set_blocksize(struct block_device *bdev, int size)
-- 
2.30.0