Re: [PATCH] dm-bufio: align write boundary on bdev_logical_block_size

[Mikulas Patocka <mpatocka@redhat.com>]

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.

Yes, but Uladzislau said that this patch doesn't work for him. So, I 
suspect that he has "logical_block_size" set incorrectly.

Mikulas

> 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.
> 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
> > 
> > 
> > 
> > From: Mikulas Patocka <mpatocka@redhat.com>
> > 
> > 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 <mpatocka@redhat.com>
> > Cc: stable@vger.kernel.org
> > 
> > ---
> >  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;
> >  
> > 
>