Re: Detecting RWF_NOWAIT support

[vcaputo]

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.

Regards,
Vito Caputo