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Date:   Mon, 16 Jan 2023 08:04:47 -0800
From:   Christoph Hellwig <hch@infradead.org>
To:     David Howells <dhowells@redhat.com>
Cc:     Jens Axboe <axboe@kernel.dk>,
        Christoph Hellwig <hch@infradead.org>,
        linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: Is there a reason why REQ_OP_READ has to be 0?
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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.