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
