Subject: Re: Detecting RWF_NOWAIT support
To: Avi Kivity <avi@scylladb.com>,
        linux-kernel <linux-kernel@vger.kernel.org>, linux-xfs@vger.kernel.org
References: <b45afd09-5fa8-74ac-4ba4-9dc9074e30cf@scylladb.com>
From: Goldwyn Rodrigues <rgoldwyn@suse.de>
Message-ID: <4ae3426d-8104-c243-72e4-671e89401b23@suse.de>
Date: Thu, 14 Dec 2017 13:15:45 -0600
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On 12/14/2017 11:38 AM, Avi Kivity wrote:
> I'm looking to add support for RWF_NOWAIT within a linux-aio iocb.
> Naturally, I need to detect at runtime whether the kernel support
> RWF_NOWAIT or not.
> 
> 
> The only method I could find was to issue an I/O with RWF_NOWAIT set,
> and look for errors. This is somewhat less than perfect:
> 
>  - from the error, I can't tell whether RWF_NOWAIT was the problem, or
> something else. If I enable a number of new features, I have to run
> through all combinations to figure out which ones are supported and
> which are not.

Here is the return codes for RWF_NOWAIT
EINVAL - not supported (older kernel)
EOPNOTSUPP - not supported
EAGAIN - supported but could not complete because I/O will be delayed
0 - supported and I/O completed (success).

> 
>  - RWF_NOWAIT support is per-filesystem, so I can't just remember not to
> enable RWF_NOWAIT globally, I have to track it per file.

Yes, the support is per filesystem. So, the application must know if the
filesystem supports it, possibly by performing a small I/O.

-- 
Goldwyn