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Message-ID: <aSQ24uN3A7PAo7VY@pc636>
Date: Mon, 24 Nov 2025 11:43:46 +0100
From: Uladzislau Rezki <urezki@...il.com>
To: Benjamin Marzinski <bmarzins@...hat.com>
Cc: Uladzislau Rezki <urezki@...il.com>, Christoph Hellwig <hch@....de>,
Mikulas Patocka <mpatocka@...hat.com>,
Alasdair Kergon <agk@...hat.com>, DMML <dm-devel@...ts.linux.dev>,
Andrew Morton <akpm@...ux-foundation.org>,
Mike Snitzer <snitzer@...hat.com>,
LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RESEND PATCH] dm-ebs: Mark full buffer dirty even on partial
write
> 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@...tvm:~# nvme id-ns -H /dev/nvme0n1 | grep npw
> > > npwg : 0
> > > npwa : 0
> > > root@...tvm:~# nvme id-ns -H /dev/nvme0n1 | grep naw
> > > nawun : 0
> > > nawupf : 0
> > > root@...tvm:~# 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@...38:~$ 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@...38:~$ cat bin/dmsetup.sh
> > #!/bin/bash
> >
> > lower=/dev/nvme0n1
> > len=$(blockdev --getsz "$lower")
> >
> > echo "0 $len ebs $lower 0 1 16" | dmsetup create nvme-8k
> > urezki@...38:~$ sudo bin/dmsetup.sh
> > urezki@...38:~$ sudo cat /sys/block/nvme0n1/queue/logical_block_size
> > 8192
> > urezki@...38:~$ sudo cat /sys/block/nvme0n1/queue/physical_block_size
> > 8192
> > urezki@...38:~$ sudo cat /sys/block/dm-0/queue/logical_block_size
> > 512
> > urezki@...38:~$ sudo cat /sys/block/dm-0/queue/physical_block_size
> > 8192
> > urezki@...38:~$ 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@...38:~$ 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@...38:~$
> >
> > with the patch:
> >
> > urezki@...38:~$ 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@...38:~$ cat bin/dmsetup.sh
> > #!/bin/bash
> >
> > lower=/dev/nvme0n1
> > len=$(blockdev --getsz "$lower")
> >
> > echo "0 $len ebs $lower 0 1 16" | dmsetup create nvme-8k
> > urezki@...38:~$ sudo bin/dmsetup.sh
> > urezki@...38:~$ sudo cat /sys/block/nvme0n1/queue/logical_block_size
> > 8192
> > urezki@...38:~$ sudo cat /sys/block/nvme0n1/queue/physical_block_size
> > 8192
> > urezki@...38:~$ sudo cat /sys/block/dm-0/queue/logical_block_size
> > 512
> > urezki@...38:~$ sudo cat /sys/block/dm-0/queue/physical_block_size
> > 8192
> > urezki@...38:~$ 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@...38:~$ sudo mount /dev/dm-0 /mnt/
> > urezki@...38:~$ 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@...38:~$
> >
> > 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
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