Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <70e4869e-1ab8-9838-5af8-8cd41a536f2b@kernel.dk>
Date:   Mon, 16 Jan 2023 09:06:28 -0700
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux aarch64; rv:102.0) Gecko/20100101
 Thunderbird/102.6.0
Subject: Re: Is there a reason why REQ_OP_READ has to be 0?
Content-Language: en-US
To:     Christoph Hellwig <hch@infradead.org>,
        David Howells <dhowells@redhat.com>
Cc:     linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <2117829.1673884910@warthog.procyon.org.uk>
 <Y8V1n4e/eeOd4n8/@infradead.org>
From:   Jens Axboe <axboe@kernel.dk>
In-Reply-To: <Y8V1n4e/eeOd4n8/@infradead.org>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 7bit
Precedence: bulk

On 1/16/23 9:04?AM, Christoph Hellwig wrote:
> On Mon, Jan 16, 2023 at 04:01:50PM +0000, David Howells wrote:
>> Hi Jens, Christoph,
>>
>> Do you know if there's a reason why REQ_OP_READ has to be 0?  I'm seeing a
>> circumstance where a direct I/O write on a blockdev is BUG'ing in my modified
>> iov_iter code because the iterator says it's a source iterator (correct), but
>> the bio->bi_opf == REQ_OP_READ (which should be wrong).
>>
>> I thought I'd move REQ_OP_READ to, say, 4 so that I could try and see if it's
>> just undefined but the kernel BUGs and then panics during boot.
> 
> There's all kind of assumptions of that from basically day 1 of
> Linux.  The most obvious one is in op_is_write, but I'm pretty sure
> there are more hidden somewhere.

I didn't get this original email...

I would not change this frivilously, as Christoph says we've used 0/1
for basic read/write data direction since basically forever. So for all
intents and purposes, yes that is basically hardwired.

-- 
Jens Axboe