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

[Uladzislau Rezki <urezki@gmail.com>]

> 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?
> 
I saw, he mentioned that and i am glad that during this discussion
we came to one more extra patch. I got the feeling that there were
misunderstanding between us, so i decided to make it more clear
that is it.

Thank you.

--
Uladzislau Rezki