Re: Detecting RWF_NOWAIT support

From: Avi Kivity
Date: Sat Dec 16 2017 - 09:49:24 EST

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.