On Mon, 17 Nov 2025, Benjamin Marzinski wrote:
> On Mon, Oct 20, 2025 at 02:48:13PM +0200, Mikulas Patocka wrote:
> >
> >
> > 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.
>
> This looks to me like it should accomplish the same thing as
> Uladzislau's patch. But I think there could still be problems with other
> dm-bufio users, for devices where the blocksize is larger than 4k.
>
> In dm_bufio_client_create() I think we want to make sure that block_size
> is a multiple of bdev_logical_block_size(bdev), instead of 512b.
I could add WARN_ON(block_size < bdev_logical_block_size(bdev)) to
dm_bufio_client_create. But I think it's too late in this development
cycle, I would add it after the next merge window closes, when I open a
new patch series for the kernel 6.20 (or 7.0).
> Otherwise block_to_sector() can return sectors that are not addressable
> on the device. Unfortunatley, I don't think all users of dm-bufio will
> pass in block_sizes that are larger than 4k (uds_make_bufio() in
> dm-vdp/indexer/io-factory.c for instance).
>
> -Ben
>
> > Please try this patch - does it fix it?
> >
> > Mikulas
I changed the patch below, so that it aligns write bios on
max3(DM_BUFIO_WRITE_ALIGN, bdev_logical_block_size(b->c->bdev),
bdev_physical_block_size(b->c->bdev)); - so that if physical block size is
greater than logical block size, the writes are aligned so that the device
doesn't do read-modify-write.
Mikulas
> > From: Mikulas Patocka <[email protected]>
> >
> > 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 <[email protected]>
> > Cc: [email protected]
> >
> > ---
> > 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;
> >
> >
>
