linux-kernel - Re: [PATCH] dm-bufio: align write boundary on bdev_logical_block

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Message-ID: <b791d8e8-5e30-b50c-04ca-e823c0a06747@redhat.com>
Date: Wed, 29 Oct 2025 11:24:25 +0100 (CET)
From: Mikulas Patocka <mpatocka@...hat.com>
To: Uladzislau Rezki <urezki@...il.com>
cc: Alasdair Kergon <agk@...hat.com>, DMML <dm-devel@...ts.linux.dev>, 
    Andrew Morton <akpm@...ux-foundation.org>, 
    Mike Snitzer <snitzer@...hat.com>, Christoph Hellwig <hch@....de>, 
    LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] dm-bufio: align write boundary on
 bdev_logical_block_size



On Tue, 28 Oct 2025, Uladzislau Rezki wrote:

> On Tue, Oct 28, 2025 at 09:47:40AM +0100, Uladzislau Rezki wrote:
> > Hello!
> > 
> > Sorry i have missed you email for unknown reason to me. It is
> > probably because you answered to email with different subject
> > i sent initially.
> > 
> > > 
> > > 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. Please try this patch - does it fix 
> > > it?
> > > 
> > If it solves what i describe i do not mind :)
> > 
> > > 
> > > 
> > > From: Mikulas Patocka <mpatocka@...hat.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@...hat.com>
> > > Cc: stable@...r.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));
> >
> Should it be physical_block_size of device? It is a min_io the device
> can perform. The point is, a user sets "ubs" size which should correspond
> to the smallest I/O the device can write, i.e. physically.

physical_block_size is unreliable - some SSDs report physical block size 
512 bytes, some 4k. Regardless of what they report, all current SSDs have 
4k sector size internally and they do slow read-modify-write cycle on 
requests that are not aligned on 4k boundary.

Mikulas

> --
> Uladzislau Rezki
>