On Fri, Aug 23, 2019 at 09:04:29AM -0300, Jason Gunthorpe wrote: > On Fri, Aug 23, 2019 at 01:23:45PM +1000, Dave Chinner wrote: > > > > But the fact that RDMA, and potentially others, can "pass the > > > pins" to other processes is something I spent a lot of time trying to work out. > > > > There's nothing in file layout lease architecture that says you > > can't "pass the pins" to another process. All the file layout lease > > requirements say is that if you are going to pass a resource for > > which the layout lease guarantees access for to another process, > > then the destination process already have a valid, active layout > > lease that covers the range of the pins being passed to it via the > > RDMA handle. > > How would the kernel detect and enforce this? There are many ways to > pass a FD. AFAIC, that's not really a kernel problem. It's more of an application design constraint than anything else. i.e. if the app passes the IB context to another process without a lease, then the original process is still responsible for recalling the lease and has to tell that other process to release the IB handle and it's resources. > IMHO it is wrong to try and create a model where the file lease exists > independently from the kernel object relying on it. In other words the > IB MR object itself should hold a reference to the lease it relies > upon to function properly. That still doesn't work. Leases are not individually trackable or reference counted objects objects - they are attached to a struct file bUt, in reality, they are far more restricted than a struct file. That is, a lease specifically tracks the pid and the _open fd_ it was obtained for, so it is essentially owned by a specific process context. Hence a lease is not able to be passed to a separate process context and have it still work correctly for lease break notifications. i.e. the layout break signal gets delivered to original process that created the struct file, if it still exists and has the original fd still open. It does not get sent to the process that currently holds a reference to the IB context. So while a struct file passed to another process might still have an active lease, and you can change the owner of the struct file via fcntl(F_SETOWN), you can't associate the existing lease with a the new fd in the new process and so layout break signals can't be directed at the lease fd.... This really means that a lease can only be owned by a single process context - it can't be shared across multiple processes (so I was wrong about dup/pass as being a possible way of passing them) because there's only one process that can "own" a struct file, and that where signals are sent when the lease needs to be broken. So, fundamentally, if you want to pass a resource that pins a file layout between processes, both processes need to hold a layout lease on that file range. And that means exclusive leases and passing layouts between processes are fundamentally incompatible because you can't hold two exclusive leases on the same file range.... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx
