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 <[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));
> > >
> > 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
