Re: [RESEND PATCH] dm-ebs: Mark full buffer dirty even on partial write

Uladzislau Rezki
2025-11-21T13:21:38Z

On Fri, Nov 21, 2025 at 08:24:21AM +0100, Christoph Hellwig wrote:
> On Thu, Nov 20, 2025 at 01:08:57PM +0100, Uladzislau Rezki wrote:
> > Could you please check below? Is the last one is correctly reported?
>
> The latter looks unexpected, but is is becase qemu is not passing through
> the qemu physical_block_size attribute to any of the nvme settings Linux
> interprets as such for NVMe (NVMe doesn't actually have the concept of
> a physical block size, unlike SCSI/ATA):
>
OK, understood and thank you for checking this.

>
> root@testvm:~# nvme id-ns -H /dev/nvme0n1 | grep npw
> npwg : 0
> npwa : 0
> root@testvm:~# nvme id-ns -H /dev/nvme0n1 | grep naw
> nawun : 0
> nawupf : 0
> root@testvm:~# nvme id-ctrl -H /dev/nvme0 | grep awupf
> awupf : 0
>
> but as said multiple times, that should not really matter - the logical
> block size is the granularity of I/O, the physical block size is just
> a performance hint.
>
Right.

As stated in commit message of the patch which is in question. 8K
emulated in qemu device with CONFIG_TRANSPARENT_HUGEPAGE=y:

urezki@pc638:~$ sudo nvme list
Node Generic SN Model Namespace Usage Format FW Rev
--------------------- --------------------- -------------------- ---------------------------------------- --------- -------------------------- ---------------- --------
/dev/nvme0n1 /dev/ng0n1 foo QEMU NVMe Ctrl 1 8.49 GB / 8.49 GB 8 KiB + 0 B 10.0.6
urezki@pc638:~$ cat bin/dmsetup.sh
#!/bin/bash

lower=/dev/nvme0n1
len=$(blockdev --getsz "$lower")

echo "0 $len ebs $lower 0 1 16" | dmsetup create nvme-8k
urezki@pc638:~$ sudo bin/dmsetup.sh
urezki@pc638:~$ sudo cat /sys/block/nvme0n1/queue/logical_block_size
8192
urezki@pc638:~$ sudo cat /sys/block/nvme0n1/queue/physical_block_size
8192
urezki@pc638:~$ sudo cat /sys/block/dm-0/queue/logical_block_size
512
urezki@pc638:~$ sudo cat /sys/block/dm-0/queue/physical_block_size
8192
urezki@pc638:~$ sudo mkfs.ext4 -F /dev/dm-0
mke2fs 1.47.0 (5-Feb-2023)
/dev/dm-0 contains a ext4 file system
last mounted on Fri Nov 21 12:22:55 2025
Discarding device blocks: done
Creating filesystem with 2072576 4k blocks and 518144 inodes
Filesystem UUID: f71adb05-c020-4406-bc0d-bdb9e5c29af7
Superblock backups stored on blocks:
32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632

Allocating group tables: done
Writing inode tables: done
Creating journal (16384 blocks): done
Writing superblocks and filesystem accounting information: mkfs.ext4: Input/output error while writing out and closing file system
urezki@pc638:~$ sudo dmesg | grep -i "i/o"
[ 71.813322] Buffer I/O error on dev dm-0, logical block 10, lost async page write
[ 71.813373] Buffer I/O error on dev dm-0, logical block 11, lost async page write
[ 71.813395] Buffer I/O error on dev dm-0, logical block 12, lost async page write
[ 71.813415] Buffer I/O error on dev dm-0, logical block 13, lost async page write
[ 71.813433] Buffer I/O error on dev dm-0, logical block 14, lost async page write
[ 71.813451] Buffer I/O error on dev dm-0, logical block 15, lost async page write
[ 71.813475] Buffer I/O error on dev dm-0, logical block 16, lost async page write
[ 71.813493] Buffer I/O error on dev dm-0, logical block 17, lost async page write
[ 71.813516] Buffer I/O error on dev dm-0, logical block 18, lost async page write
[ 71.813537] Buffer I/O error on dev dm-0, logical block 19, lost async page write
urezki@pc638:~$

with the patch:

urezki@pc638:~$ sudo nvme list
Node Generic SN Model Namespace Usage Format FW Rev
--------------------- --------------------- -------------------- ---------------------------------------- --------- -------------------------- ---------------- --------
/dev/nvme0n1 /dev/ng0n1 foo QEMU NVMe Ctrl 1 8.49 GB / 8.49 GB 8 KiB + 0 B 10.0.6
urezki@pc638:~$ cat bin/dmsetup.sh
#!/bin/bash

lower=/dev/nvme0n1
len=$(blockdev --getsz "$lower")

echo "0 $len ebs $lower 0 1 16" | dmsetup create nvme-8k
urezki@pc638:~$ sudo bin/dmsetup.sh
urezki@pc638:~$ sudo cat /sys/block/nvme0n1/queue/logical_block_size
8192
urezki@pc638:~$ sudo cat /sys/block/nvme0n1/queue/physical_block_size
8192
urezki@pc638:~$ sudo cat /sys/block/dm-0/queue/logical_block_size
512
urezki@pc638:~$ sudo cat /sys/block/dm-0/queue/physical_block_size
8192
urezki@pc638:~$ sudo mkfs.ext4 -F /dev/dm-0
mke2fs 1.47.0 (5-Feb-2023)
Discarding device blocks: done
Creating filesystem with 2072576 4k blocks and 518144 inodes
Filesystem UUID: c7dff4c7-aa7e-4c94-98ee-f9ea2da92a06
Superblock backups stored on blocks:
32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632

Allocating group tables: done
Writing inode tables: done
Creating journal (16384 blocks): done
Writing superblocks and filesystem accounting information: done

urezki@pc638:~$ sudo mount /dev/dm-0 /mnt/
urezki@pc638:~$ ls -al /mnt/
total 24
drwxr-xr-x 3 root root 4096 Nov 21 12:22 .
drwxr-xr-x 19 root root 4096 Jul 10 19:42 ..
drwx------ 2 root root 16384 Nov 21 12:22 lost+found
urezki@pc638:~$

How do we solve this?

Mikulas proposed to use below patch:

<snip>
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;
<snip>

and it fixes the setup which i described in the commit message, but i
have question.

Why in dm-ebs we need to offload partial buffer < ubf size?

Thank you for answers!

--
Uladzislau Rezki