linux-kernel - Re: [PATCH] ublk: skip blk_mq_tag_to

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Message-ID: <Z_eOX-8QHxsq21Rz@infradead.org>
Date: Thu, 10 Apr 2025 02:24:47 -0700
From: Christoph Hellwig <hch@...radead.org>
To: Caleb Sander Mateos <csander@...estorage.com>
Cc: Ming Lei <ming.lei@...hat.com>, Jens Axboe <axboe@...nel.dk>,
	linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ublk: skip blk_mq_tag_to_rq() bounds check

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.