[PATCH] dm-bufio: align write boundary on bdev_logical_block_size

[Mikulas Patocka <mpatocka@redhat.com>]

On Mon, 20 Oct 2025, Uladzislau Rezki (Sony) wrote:

> When performing a read-modify-write(RMW) operation, any modification
> to a buffered block must cause the entire buffer to be marked dirty.
> 
> Marking only a subrange as dirty is incorrect because the underlying
> device block size(ubs) defines the minimum read/write granularity. A
> lower device can perform I/O only on regions which are fully aligned
> and sized to ubs.

Hi

I think it would be better to fix this in dm-bufio, so that other dm-bufio 
users would also benefit from the fix. Please try this patch - does it fix 
it?

Mikulas



From: Mikulas Patocka <mpatocka@redhat.com>

There may be devices with logical block size larger than 4k. Fix
dm-bufio, so that it will align I/O on logical block size. This commit
fixes I/O errors on the dm-ebs target on the top of emulated nvme device
with 8k logical block size created with qemu parameters:

-device nvme,drive=drv0,serial=foo,logical_block_size=8192,physical_block_size=8192

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Cc: stable@vger.kernel.org

---
 drivers/md/dm-bufio.c |    9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

Index: linux-2.6/drivers/md/dm-bufio.c
===================================================================
--- linux-2.6.orig/drivers/md/dm-bufio.c	2025-10-13 21:42:47.000000000 +0200
+++ linux-2.6/drivers/md/dm-bufio.c	2025-10-20 14:40:32.000000000 +0200
@@ -1374,7 +1374,7 @@ static void submit_io(struct dm_buffer *
 {
 	unsigned int n_sectors;
 	sector_t sector;
-	unsigned int offset, end;
+	unsigned int offset, end, align;
 
 	b->end_io = end_io;
 
@@ -1388,9 +1388,10 @@ static void submit_io(struct dm_buffer *
 			b->c->write_callback(b);
 		offset = b->write_start;
 		end = b->write_end;
-		offset &= -DM_BUFIO_WRITE_ALIGN;
-		end += DM_BUFIO_WRITE_ALIGN - 1;
-		end &= -DM_BUFIO_WRITE_ALIGN;
+		align = max(DM_BUFIO_WRITE_ALIGN, bdev_logical_block_size(b->c->bdev));
+		offset &= -align;
+		end += align - 1;
+		end &= -align;
 		if (unlikely(end > b->c->block_size))
 			end = b->c->block_size;