Re: [PATCH v4 09/12] file-posix: base the zone append limit on the transfer limit

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Mon, Sep 07, 2026 at 01:07:44PM +0200, Niklas Cassel wrote:
> The zone_append_max_bytes queue attribute is the largest
> REQ_OP_ZONE_APPEND that the device accepts, and Linux has no interface
> for issuing one from userspace: include/uapi has no such operation, and
> every submitter of REQ_OP_ZONE_APPEND is in the kernel. Userspace writes
> to a sequential zone with an ordinary write at the write pointer.
> 
> That is what this driver does. raw_co_zone_append() substitutes the write
> pointer of the zone for the offset and hands the request to raw_co_prw(),
> which reaches handle_aiocb_rw_vector() and issues a plain pwritev(). The
> kernel never sees a zone append, so the attribute describes a limit on an
> operation that is never issued.
> 
> Nor is it a limit that this driver runs into. The kernel splits a write
> that exceeds the transfer limit rather than refusing it, so a larger
> append succeeds: on a null_blk device whose zone_append_max_bytes is
> 130560, a 16 MiB append completes and advances the write pointer by
> 16 MiB. Reporting the attribute only understates what the driver can do,
> because Linux derives it as a minimum that already includes max_sectors
> and chunk_sectors.
> 
> Report max_hw_transfer instead, the limit that governs the write the
> driver actually issues. There is no use in telling a guest that it may
> append more than the device carries in one command, and a frontend then
> does not have to reason about how this driver implements an append in
> order to bound the value it advertises.
> 
> On a null_blk device with max_hw_sectors_kb of 127 and
> zone_append_max_bytes of 130560, virtio-blk reports 255 sectors before
> and after.
> 
> Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
> Signed-off-by: Niklas Cassel <cassel@kernel.org>
> ---
>  block/file-posix.c | 13 +++++++++----
>  1 file changed, 9 insertions(+), 4 deletions(-)

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

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