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

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Message-ID: <aQIRV51yLQzOwJvB@pc636>
Date: Wed, 29 Oct 2025 14:06:31 +0100
From: Uladzislau Rezki <urezki@...il.com>
To: Mikulas Patocka <mpatocka@...hat.com>
Cc: Uladzislau Rezki <urezki@...il.com>, 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 Wed, Oct 29, 2025 at 11:24:25AM +0100, Mikulas Patocka wrote:
> 
> 
> 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.
> 
I see. Some NVMEs have buggy firmwares therefore we have a lot of quicks
flags. I agree there is mess there.
 
The change does not help my project and case. I posted the patch to fix
the dm-ebs as the code offloads partial size instead of ubs size, what
actually a user asking for. When a target is created, the physical_block_size
corresponds to ubs.
 
I really appreciate if you take the fix i posted. Your patch can be
sent out separately.
 
Does it work for you?
 
Thank you!
 
--
Uladzislau Rezki