Re: [PATCH] ublk: skip blk_mq_tag_to_rq() bounds check

[Christoph Hellwig]

On Tue, Apr 08, 2025 at 08:49:54PM -0600, Caleb Sander Mateos wrote:
> The ublk driver calls blk_mq_tag_to_rq() in several places.
> blk_mq_tag_to_rq() tolerates an invalid tag for the tagset, checking it
> against the number of tags and returning NULL if it is out of bounds.
> But all the calls from the ublk driver have already verified the tag
> against the ublk queue's queue depth. In ublk_commit_completion(),
> ublk_handle_need_get_data(), and case UBLK_IO_COMMIT_AND_FETCH_REQ, the
> tag has already been checked in __ublk_ch_uring_cmd(). In
> ublk_abort_queue(), the loop bounds the tag by the queue depth. In
> __ublk_check_and_get_req(), the tag has already been checked in
> __ublk_ch_uring_cmd(), in the case of ublk_register_io_buf(), or in
> ublk_check_and_get_req().
> 
> So just index the tagset's rqs array directly in the ublk driver.
> Convert the tags to unsigned, as blk_mq_tag_to_rq() does.

Poking directly into block layer internals feels like a really bad
idea.  If this is important enough we'll need a non-checking helper
in the core code, but as with all these kinds of micro-optimizations
it better have a really good justification.