Re: Detecting RWF_NOWAIT support

[Avi Kivity]

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:
> > > >
> > > >  ÂÂ- 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).
> > >
> > > >  ÂÂ- 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.