linux-kernel - Re: Detecting RWF

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Message-ID: <0ae3b9d3-0e57-5562-5a8d-62496a261521@scylladb.com>
Date:   Sat, 16 Dec 2017 16:49:08 +0200
From:   Avi Kivity <avi@...lladb.com>
To:     Goldwyn Rodrigues <rgoldwyn@...e.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-xfs@...r.kernel.org
Subject: Re: Detecting RWF_NOWAIT support



On 12/14/2017 09:15 PM, Goldwyn Rodrigues wrote:
>
> 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

Which of these are returned from io_submit() and which are returned in 
the iocb?

> 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.

So the application must know about filesystem mount points, and be 
prepared to create a file and try to write it (in case the filesystem is 
empty) or alter its behavior during runtime depending on the errors it sees.