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

Benjamin Marzinski
2025-11-21T16:48:54Z

On Fri, Nov 21, 2025 at 02:21:34PM +0100, Uladzislau Rezki wrote:
> 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?

Um, did you notice that Mikulas accepted your patch?

>
> Thank you for answers!
>
> --
> Uladzislau Rezki