linux-kernel - Detecting RWF

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Date:   Thu, 14 Dec 2017 19:38:34 +0200
From:   Avi Kivity <avi@...lladb.com>
To:     Goldwyn Rodrigues <rgoldwyn@...e.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-xfs@...r.kernel.org
Subject: Detecting RWF_NOWAIT support

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.

  - 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.

Did I miss another method of detecting RWF_NOWAIT?