Re: [RFC v2 0/6] block/io: avoid failure caused by misaligned BLKZEROOUT ioctl

[Stefan Hajnoczi <stefanha@redhat.com>]

[-- Attachment #1: Type: text/plain, Size: 1337 bytes --]

On Fri, Jan 09, 2026 at 01:08:27PM +0100, Fiona Ebner wrote:
> Previous discussion here:
> https://lore.kernel.org/qemu-devel/20260105143416.737482-1-f.ebner@proxmox.com/
> 
> Commit 5634622bcb ("file-posix: allow BLKZEROOUT with -t writeback")
> enables the BLKZEROOUT ioctl when using 'writeback' cache, regressing
> certain 'qemu-img convert' invocations, because of a pre-existing
> issue. Namely, the BLKZEROOUT ioctl might fail with errno EINVAL when
> the request is shorter than the block size of the block device.
> 
> Stefan suggested prioritizing bl.pwrite_zeroes_alignment in
> bdrv_co_do_zero_pwritev(). This RFC explores that approach and the
> issues with qcow2 I encountered, where
> bl.pwrite_zeroes_alignment = s->subcluster_size;
> I would be happy to discuss potential solutions and whether we should
> use this approach after all.

Hi Fiona,
I wanted to continue this discussion. My thoughts are that making
bdrv_co_do_zero_pwritev() use bl.pwrite_zeroes_alignment is the right
long-term solution to keep all the padding logic in one place.

On the other hand, your series shows it involves fixing a bunch of test
failures and that's not fun. The original bug that is being solved here
is my doing, so feel free to hand this over to me if you decide you
don't want to work on it.

Stefan

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 488 bytes --]