Re: Detecting RWF_NOWAIT support

[Avi Kivity]

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:
> >
> >  Â- 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).
>
> >  Â- 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.