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.

Ex biosnoop log. Device lba size 512, optimal size 4k:
Before:
Time        Process        PID     Device      LBA        Size      Lat
0.843734    md0_raid10     5267    nvme0n1   W 24         3584      1.17
0.843933    md0_raid10     5267    nvme1n1   W 24         3584      1.36
0.843968    md0_raid10     5267    nvme1n1   W 14207939968 4096      0.01
0.843979    md0_raid10     5267    nvme0n1   W 14207939968 4096      0.02

After:
Time        Process        PID     Device      LBA        Size      Lat
18.374244   md0_raid10     6559    nvme0n1   W 24         4096      0.01
18.374253   md0_raid10     6559    nvme1n1   W 24         4096      0.01
18.374300   md0_raid10     6559    nvme0n1   W 11020272296 4096      0.01
18.374306   md0_raid10     6559    nvme1n1   W 11020272296 4096      0.02

Signed-off-by: Jon Derrick <jonathan.derrick@linux.dev>
---
 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 {