Re: [PATCH] fs: Return EOPNOTSUPP if block layer does not support REQ_NOWAIT

[Goldwyn Rodrigues <rgoldwyn@suse.de>]

On  8:35 17/12, Dave Chinner wrote:
> > 
> > I was not too happy getting it all the way down to block layer either.
> > The multi-devices makes it worse. However, here we are and we need to
> > tell the user that RWF_NOWAIT is not supported in this environment.
> 
> RWF_NOWAIT matters for filesystems much more than the underlying
> block device. If the application is accessing the blockd evice
> directly, then yes, RWF_NOWAIT support in the block device matters.
> But when the IO is being done through the filesystem it's far more
> important to avoid blocking on filesystem locks that whatever the
> block device does....
> 
> Hence I think that if the bio is coming from a filesystem,
> REQ_NOWAIT should always be accepted or bounced with EAGAIN and
> never failed with EOPNOTSUPP. It just makes no sense at all for
> filesytsem based IO....

It was initially suggested where the block layer would retry getting
a bio in get_request(). While request based devices were fine, the bio
based ones such as MD needed extra work. However, when I actually got
down to writing code for multi-device, it got more hurdles than
solutions primarily in the area of bio merging.

RWF_NOWAIT should have been restricted to filesystems and I think we
should do away (or at least ignore) REQ_NOWAIT for now.

-- 
Goldwyn