Re: bcachefs status update (it's done cooking; let's get this sucker merged)

[Kent Overstreet]

On Tue, Jun 11, 2019 at 02:33:36PM +1000, Dave Chinner wrote:
> I just recently said this with reference to the range lock stuff I'm
> working on in the background:
> 
> 	FWIW, it's to avoid problems with stupid userspace stuff
> 	that nobody really should be doing that I want range locks
> 	for the XFS inode locks.  If userspace overlaps the ranges
> 	and deadlocks in that case, they they get to keep all the
> 	broken bits because, IMO, they are doing something
> 	monumentally stupid. I'd probably be making it return
> 	EDEADLOCK back out to userspace in the case rather than
> 	deadlocking but, fundamentally, I think it's broken
> 	behaviour that we should be rejecting with an error rather
> 	than adding complexity trying to handle it.
> 
> So I think this recusive locking across a page fault case should
> just fail, not add yet more complexity to try to handle a rare
> corner case that exists more in theory than in reality. i.e put the
> lock context in the current task, then if the page fault requires a
> conflicting lock context to be taken, we terminate the page fault,
> back out of the IO and return EDEADLOCK out to userspace. This works
> for all types of lock contexts - only the filesystem itself needs to
> know what the lock context pointer contains....

Ok, I'm totally on board with returning EDEADLOCK.

Question: Would we be ok with returning EDEADLOCK for any IO where the buffer is
in the same address space as the file being read/written to, even if the buffer
and the IO don't technically overlap?

This would simplify things a lot and eliminate a really nasty corner case - page
faults trigger readahead. Even if the buffer and the direct IO don't overlap,
readahead can pull in pages that do overlap with the dio.

And on getting EDEADLOCK we could fall back to buffered IO, so userspace would
never know...