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: > > > > > > > > - 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. Cheers, Vito Caputo -- To unsubscribe from this list: send the line "unsubscribe linux-xfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
