linux-kernel - Re: [RESEND PATCH] dm-ebs: Mark full buffer dirty even on partial write

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Message-ID: <aR8E2ZZtOi0RZt06@pc636>
Date: Thu, 20 Nov 2025 13:08:57 +0100
From: Uladzislau Rezki <urezki@...il.com>
To: Christoph Hellwig <hch@....de>, Mikulas Patocka <mpatocka@...hat.com>
Cc: Mikulas Patocka <mpatocka@...hat.com>,
	Benjamin Marzinski <bmarzins@...hat.com>,
	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>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RESEND PATCH] dm-ebs: Mark full buffer dirty even on partial
 write

On Thu, Nov 20, 2025 at 07:21:46AM +0100, Christoph Hellwig wrote:
> On Wed, Nov 19, 2025 at 06:26:13PM +0100, Mikulas Patocka wrote:
> > 
> > 
> > On Wed, 19 Nov 2025, Christoph Hellwig wrote:
> > 
> > > On Tue, Nov 18, 2025 at 06:21:56PM +0100, Mikulas Patocka wrote:
> > > > OK - I accepted Uladzislau's patch. As logical block size and physical 
> > > > block size seem to be unreliable, it's better to set the size in dm-ebs.
> > > 
> > > logical and physical block size are reliable.  Uladzislau just seems
> > > to have a completely broken device that needs fixing, because it will
> > 
> > He created a qemu-emulated NVMe device with physical and logical block 
> > size 8192 in a virtual machine. And logical block size was reported as 512 
> > in the guest kernel - so it is either a qemu bug or a kernel bug.
> 
> No, that's not the case.  If you use his command line you'll see a qemu
> device with 8192 logical blocks assuming you have support for large
> folios, or a completely unusuable device that claims to have 512
> byte blocks for compatibility, but also a capacity of zero so that no
> one can use it for anything but passthrough.
> 
> > in nvme_update_disk_info there is this piece of code:
> >         if (blk_validate_block_size(bs)) {
> >                 bs = (1 << 9);
> >                 valid = false;
> >         }
> 
> Yes, that's what I mentioned above.  The valid=false sets the capacity
> to zero, so you're not actually going to be able to use this device.
> 
> > So, the valid block size depends on whether CONFIG_TRANSPARENT_HUGEPAGE is 
> > defined, which is quite weird.
> 
> And also the page size, and none of that is too weird.  You need support
> efficiently allocating large order folios to support a
> block size > PAGE_SIZE and currently CONFIG_TRANSPARENT_HUGEPAGE is
> the guard for that.  There was some talk of lifting that, but that
> requires a bit of work.
> 
Could you please check below? Is the last one is correctly reported?
I have enabled the CONFIG_TRANSPARENT_HUGEPAGE option. If i specify,
8192, 8192 first case, reports are what i set. Second variant 512, 8129
shows 512, 512:

CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE=y
CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD=y
CONFIG_TRANSPARENT_HUGEPAGE=y
CONFIG_TRANSPARENT_HUGEPAGE_ALWAYS=y
# CONFIG_TRANSPARENT_HUGEPAGE_MADVISE is not set
# CONFIG_TRANSPARENT_HUGEPAGE_NEVER is not set

-device nvme,drive=drv0,serial=foo,logical_block_size=8192,physical_block_size=8192
urezki@...38:~$ sudo nvme list
Node                  Generic               SN                   Model                                    Namespace Usage                      Format           FW Rev
--------------------- --------------------- -------------------- ---------------------------------------- --------- -------------------------- ---------------- --------
/dev/nvme0n1          /dev/ng0n1            foo                  QEMU NVMe Ctrl                           1           8.49  GB /   8.49  GB      8 KiB +  0 B   10.0.6
urezki@...38:~$ sudo cat /sys/block/nvme0n1/queue/logical_block_size
8192
urezki@...38:~$ sudo cat /sys/block/nvme0n1/queue/physical_block_size
8192
urezki@...38:~$


-device nvme,drive=drv0,serial=foo,logical_block_size=512,physical_block_size=8192
urezki@...38:~$ sudo nvme list
Node                  Generic               SN                   Model                                    Namespace Usage                      Format           FW Rev
--------------------- --------------------- -------------------- ---------------------------------------- --------- -------------------------- ---------------- --------
/dev/nvme0n1          /dev/ng0n1            foo                  QEMU NVMe Ctrl                           1           8.49  GB /   8.49  GB    512   B +  0 B   10.0.6
urezki@...38:~$ sudo cat /sys/block/nvme0n1/queue/physical_block_size
512
urezki@...38:~$ sudo cat /sys/block/nvme0n1/queue/logical_block_size
512
urezki@...38:~$

Is that what should be reported?

--
Uladzislau Rezki