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

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Message-ID: <a76ac487-564e-4b6e-89fb-9c848a398c43@kernel.dk>
Date: Thu, 10 Apr 2025 07:13:06 -0600
From: Jens Axboe <axboe@...nel.dk>
To: Christoph Hellwig <hch@...radead.org>,
 Caleb Sander Mateos <csander@...estorage.com>
Cc: Ming Lei <ming.lei@...hat.com>, linux-block@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ublk: skip blk_mq_tag_to_rq() bounds check

On 4/10/25 3:24 AM, Christoph Hellwig wrote:
> 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.

FWIW, I agree, and I also have a hard time imagining this making much of
a measurable difference. Caleb, was this based "well this seems
pointless" or was it something you noticed in profiling/testing?

-- 
Jens Axboe