[PATCH v2] md: Use optimal I/O size for last bitmap page

[Jonathan Derrick <jonathan.derrick@linux.dev>]

From: Jon Derrick <jonathan.derrick@linux.dev>

If the bitmap space has enough room, size the I/O for the last bitmap
page write to the optimal I/O size for the storage device. The expanded
write is checked that it won't overrun the data or metadata.

This change helps increase performance by preventing unnecessary
device-side read-mod-writes due to non-atomic write unit sizes.

Example Intel/Solidigm P5520
Raid10, Chunk-size 64M, bitmap-size 57228 bits

$ mdadm --create /dev/md0 --level=10 --raid-devices=4 /dev/nvme{0,1,2,3}n1 --assume-clean --bitmap=internal --bitmap-chunk=64M
$ fio --name=test --direct=1 --filename=/dev/md0 --rw=randwrite --bs=4k --runtime=60

Without patch:
  write: IOPS=1676, BW=6708KiB/s (6869kB/s)(393MiB/60001msec); 0 zone resets

With patch:
  write: IOPS=15.7k, BW=61.4MiB/s (64.4MB/s)(3683MiB/60001msec); 0 zone resets

Biosnoop:
Without patch:
Time        Process        PID     Device      LBA        Size      Lat
1.410377    md0_raid10     6900    nvme0n1   W 16         4096      0.02
1.410387    md0_raid10     6900    nvme2n1   W 16         4096      0.02
1.410374    md0_raid10     6900    nvme3n1   W 16         4096      0.01
1.410381    md0_raid10     6900    nvme1n1   W 16         4096      0.02
1.410411    md0_raid10     6900    nvme1n1   W 115346512  4096      0.01
1.410418    md0_raid10     6900    nvme0n1   W 115346512  4096      0.02
1.410915    md0_raid10     6900    nvme2n1   W 24         3584      0.43
1.410935    md0_raid10     6900    nvme3n1   W 24         3584      0.45
1.411124    md0_raid10     6900    nvme1n1   W 24         3584      0.64
1.411147    md0_raid10     6900    nvme0n1   W 24         3584      0.66
1.411176    md0_raid10     6900    nvme3n1   W 2019022184 4096      0.01
1.411189    md0_raid10     6900    nvme2n1   W 2019022184 4096      0.02

With patch:
Time        Process        PID     Device      LBA        Size      Lat
5.747193    md0_raid10     727     nvme0n1   W 16         4096      0.01
5.747192    md0_raid10     727     nvme1n1   W 16         4096      0.02
5.747195    md0_raid10     727     nvme3n1   W 16         4096      0.01
5.747202    md0_raid10     727     nvme2n1   W 16         4096      0.02
5.747229    md0_raid10     727     nvme3n1   W 1196223704 4096      0.02
5.747224    md0_raid10     727     nvme0n1   W 1196223704 4096      0.01
5.747279    md0_raid10     727     nvme0n1   W 24         4096      0.01
5.747279    md0_raid10     727     nvme1n1   W 24         4096      0.02
5.747284    md0_raid10     727     nvme3n1   W 24         4096      0.02
5.747291    md0_raid10     727     nvme2n1   W 24         4096      0.02
5.747314    md0_raid10     727     nvme2n1   W 2234636712 4096      0.01
5.747317    md0_raid10     727     nvme1n1   W 2234636712 4096      0.02

Signed-off-by: Jon Derrick <jonathan.derrick@linux.dev>
---
v1->v2: Add more information to commit message

 drivers/md/md-bitmap.c | 27 ++++++++++++++++++---------
 1 file changed, 18 insertions(+), 9 deletions(-)

diff --git a/drivers/md/md-bitmap.c b/drivers/md/md-bitmap.c
index e7cc6ba1b657..569297ea9b99 100644
--- a/drivers/md/md-bitmap.c
+++ b/drivers/md/md-bitmap.c
@@ -220,6 +220,7 @@ static int write_sb_page(struct bitmap *bitmap, struct page *page, int wait)
 	rdev = NULL;
 	while ((rdev = next_active_rdev(rdev, mddev)) != NULL) {
 		int size = PAGE_SIZE;
+		int optimal_size = PAGE_SIZE;
 		loff_t offset = mddev->bitmap_info.offset;
 
 		bdev = (rdev->meta_bdev) ? rdev->meta_bdev : rdev->bdev;
@@ -228,9 +229,14 @@ static int write_sb_page(struct bitmap *bitmap, struct page *page, int wait)
 			int last_page_size = store->bytes & (PAGE_SIZE-1);
 			if (last_page_size == 0)
 				last_page_size = PAGE_SIZE;
-			size = roundup(last_page_size,
-				       bdev_logical_block_size(bdev));
+			size = roundup(last_page_size, bdev_logical_block_size(bdev));
+			if (bdev_io_opt(bdev) > bdev_logical_block_size(bdev))
+				optimal_size = roundup(last_page_size, bdev_io_opt(bdev));
+			else
+				optimal_size = size;
 		}
+
+
 		/* Just make sure we aren't corrupting data or
 		 * metadata
 		 */
@@ -246,9 +252,11 @@ static int write_sb_page(struct bitmap *bitmap, struct page *page, int wait)
 				goto bad_alignment;
 		} else if (offset < 0) {
 			/* DATA  BITMAP METADATA  */
-			if (offset
-			    + (long)(page->index * (PAGE_SIZE/512))
-			    + size/512 > 0)
+			loff_t off = offset + (long)(page->index * (PAGE_SIZE/512));
+			if (size != optimal_size &&
+			    off + optimal_size/512 <= 0)
+				size = optimal_size;
+			else if (off + size/512 > 0)
 				/* bitmap runs in to metadata */
 				goto bad_alignment;
 			if (rdev->data_offset + mddev->dev_sectors
@@ -257,10 +265,11 @@ static int write_sb_page(struct bitmap *bitmap, struct page *page, int wait)
 				goto bad_alignment;
 		} else if (rdev->sb_start < rdev->data_offset) {
 			/* METADATA BITMAP DATA */
-			if (rdev->sb_start
-			    + offset
-			    + page->index*(PAGE_SIZE/512) + size/512
-			    > rdev->data_offset)
+			loff_t off = rdev->sb_start + offset + page->index*(PAGE_SIZE/512);
+			if (size != optimal_size &&
+			    off + optimal_size/512 <= rdev->data_offset)
+				size = optimal_size;
+			else if (off + size/512 > rdev->data_offset)
 				/* bitmap runs in to data */
 				goto bad_alignment;
 		} else {
-- 
2.27.0