Re: [PATCH 12/13] block: add blkdebug events for a write before it is tracked

[Eric Blake <eblake@redhat.com>]

On Wed, Sep 30, 2026 at 06:49:58PM +0200, Denis V. Lunev wrote:
> From: Denis V. Lunev <den@openvz.org>
> 
> A breakpoint on pwritev or pwritev_zero stops a request that has
> already joined the list of tracked requests, so it is ordered against
> everything that arrives later. There was no way to hold a request in
> the window before that, which is where a filter has published its
> state but the block layer does not yet know the request exists.
> 
> Fire pwritev_untracked and pwritev_zero_untracked from
> bdrv_co_pwritev_part() once the request counts as in flight and before
> tracked_request_begin(). They come in a pair like the events they
> mirror, so that a test can tell a preallocation from the writes racing
> with it: blkdebug matches on the event, not on the request.
> 
> Signed-off-by: Denis V. Lunev <den@openvz.org>
> CC: Kevin Wolf <kwolf@redhat.com>
> CC: Hanna Reitz <hreitz@redhat.com>
> CC: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
> CC: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
> CC: Eric Blake <eblake@redhat.com>
> CC: Markus Armbruster <armbru@redhat.com>
> ---
>  block/io.c           |  3 +++
>  qapi/block-core.json | 10 +++++++++-
>  2 files changed, 12 insertions(+), 1 deletion(-)
> 

> +++ b/qapi/block-core.json
> @@ -3897,6 +3897,13 @@
>  #
>  # @none: triggers once at creation of the blkdebug node (since 4.1)
>  #
> +# @pwritev_untracked: a write, before it joins the list of tracked
> +#     requests and so before it is ordered against any other request
> +#     (since 11.2)
> +#
> +# @pwritev_zero_untracked: the same for a write zeroes request
> +#     (since 11.2)

I don't know if Markus has any guidance to whether the text for one
parameter can reliably refer to an earlier one. But I'm assuming that
the generated docs list things sequentially by declaration order
rather than reordering to alphabetical or randomized hash layout
ordering, so the shorter second text is probably fine.

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.
Virtualization:  qemu.org | libguestfs.org