linux-ext4 - Re: [PATCH 1/8] nowait aio: Introduce IOCB_FLAG

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Date:   Wed, 1 Mar 2017 14:44:21 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Goldwyn Rodrigues <rgoldwyn@...e.de>
Cc:     Christoph Hellwig <hch@...radead.org>, jack@...e.com,
        linux-fsdevel@...r.kernel.org, linux-block@...r.kernel.org,
        linux-btrfs@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-xfs@...r.kernel.org
Subject: Re: [PATCH 1/8] nowait aio: Introduce IOCB_FLAG_NOWAIT

On Wed, Mar 01, 2017 at 10:57:17AM -0600, Goldwyn Rodrigues wrote:
> RWF_* ? Isn't that kernel space flags? Or did you intend to say
> IOCB_FLAG_*?

No, they are the flags for preadv2/pwritev2.

> If yes, we maintain two flag fields? aio_reserved1 (perhaps
> renamed to aio_flags2) and aio_flags?

Yes - I'd call it aio_rw_flags or similar.

> aio_reserved1 is also used to return key for the purpose of io_cancel,
> but we should be able to fetch the flags before putting the key value
> there. Still I am not comfortable using the same field for it because it
> will be overwritten when io_submit returns.

It's not - the key is a separate field.  It's just that the two are
defined using a very strange macro switching around their positions
based on the endiannes.

> Which brings me to the next question: What is the purpose of aio_key?
> Why is aio_key set to KIOCB_KEY (which is zero) every time? You are not
> differentiating the request by setting all the iocb's key to zero.

I don't know the history of this rather odd field.