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SubjectRe: Detecting RWF_NOWAIT support

On 12/16/2017 08:12 PM, wrote:
> On Sat, Dec 16, 2017 at 10:03:38AM -0800, wrote:
>> On Sat, Dec 16, 2017 at 04:49:08PM +0200, 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?
>>>> 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.
>> Can't the application simply add a "nowait" flag to its open file
>> descriptor encapsulation struct, then in the constructor perform a
>> zero-length RWF_NOWAIT write immediately after opening the fd to set the
>> flag? Then all writes branch according to the flag.
>> According to write(2):
>> If count is zero and fd refers to a regular file, then write()
>> may return a failure status if one of the errors below is
>> detected. If no errors are detected, or error detection is not
>> performed, 0 will be returned without causing any other effect.
>> If count is zero and fd refers to a file other than a regular
>> file, the results are not specified.
>> So the zero-length RWF_NOWAIT write should return zero, unless it's not
>> supported.
> Oh, I assumed this new flag applied to pwritev2() flags. Disregard my
> comment, I see the ambiguity causing your question Avi and do not know
> the best approach.

Actually it's not a bad idea. I'm using AIO, not p{read,write}v2, but I
can assume that the response will be the same and that a zero-length
read will return immediately.

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