Re: Detecting RWF_NOWAIT support

[Avi Kivity <avi@scylladb.com>]

On 12/18/2017 05:28 AM, Goldwyn Rodrigues wrote:
>
> On 12/16/2017 08:49 AM, Avi Kivity wrote:
>>
>> 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?
> These are returned in iocb.

Thanks.

>
>>> 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.
> Well yes. Hopefully, the application knows what it is doing when it
> performs RWF_NOWAIT.

This type of interface makes it very hard to consume new kernel 
facilities in a backward compatible way. The kernel should advertise 
what support it provides; for example this support could be advertised 
via statx(2).

For examples of facilities that advertise their capabilities, see 
membarrier(2) and KVM.