Re: Detecting RWF_NOWAIT support

[Avi Kivity]

On 12/16/2017 08:12 PM, vcaputo@xxxxxxxxxxx wrote:
> On Sat, Dec 16, 2017 at 10:03:38AM -0800, vcaputo@xxxxxxxxxxx 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:
> > > > >
> > > > >   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.
> > 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.