Re: [PATCH 4/6] block/io: safeguard max transfer calculation in bdrv_aligned_pwritev()

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Fri, Jan 09, 2026 at 01:08:31PM +0100, Fiona Ebner wrote:
> This partially fixes iotest 177 with qcow2, where max_transfer is
> 64KiB, but the cluster size and thus pwrite_zeroes_alignment is 1MiB.
> Previously, max_transfer would be calculated as 0, triggering an
> assertion later.
> 
> Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
> ---
>  block/io.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/block/io.c b/block/io.c
> index 12dc153573..233b2617ea 100644
> --- a/block/io.c
> +++ b/block/io.c
> @@ -2087,8 +2087,10 @@ bdrv_aligned_pwritev(BdrvChild *child, BdrvTrackedRequest *req,
>      assert(is_power_of_2(align));
>      assert((offset & (align - 1)) == 0);
>      assert((bytes & (align - 1)) == 0);
> -    max_transfer = QEMU_ALIGN_DOWN(MIN_NON_ZERO(bs->bl.max_transfer, INT_MAX),
> -                                   align);
> +    max_transfer = MIN_NON_ZERO(bs->bl.max_transfer, INT_MAX);
> +    if (max_transfer > align) {
> +        max_transfer = QEMU_ALIGN_DOWN(max_transfer, align);
> +    }

max_transfer < align seems paradoxical. It's a situation where the
largest allowed I/O request cannot meet alignment requirements.

Every other place that uses max_transfer in QEMU would also need to cope
with this.

block/blkdebug.c:blkdebug_open() fails if max_transfer is not aligned,
indicating that there are assumptions at least in some places that
max_transfer is aligned.

I hesitate to make this change because I fear it will break more things.
Why was max_transfer 64KB while pwrite_zeroes_alignment was 1MiB? Either
max_transfer should fit the alignment or pwrite_zeroes code needs to
distinguish between actual write zeroes operations and read-write-modify
I/O (which is plain read/write and not subject to
pwrite_zeroes_alignment).

Stefan

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