Re: [RFC PATCH v2 00/19] RDMA/FS DAX truncate proposal V1,000,002 ; -)

On Fri, Aug 23, 2019 at 10:59:14AM +1000, Dave Chinner wrote: > On Wed, Aug 21, 2019 at 11:02:00AM -0700, Ira Weiny wrote: > > On Tue, Aug 20, 2019 at 08:55:15AM -0300, Jason Gunthorpe wrote: > > > On Tue, Aug 20, 2019 at 11:12:10AM +1000, Dave Chinner wrote: > > > > On Mon, Aug 19, 2019 at 09:38:41AM -0300, Jason Gunthorpe wrote: > > > > > On Mon, Aug 19, 2019 at 07:24:09PM +1000, Dave Chinner wrote: > > > > > > > > > > > So that leaves just the normal close() syscall exit case, where the > > > > > > application has full control of the order in which resources are > > > > > > released. We've already established that we can block in this > > > > > > context. Blocking in an interruptible state will allow fatal signal > > > > > > delivery to wake us, and then we fall into the > > > > > > fatal_signal_pending() case if we get a SIGKILL while blocking. > > > > > > > > > > The major problem with RDMA is that it doesn't always wait on close() for the > > > > > MR holding the page pins to be destoyed. This is done to avoid a > > > > > deadlock of the form: > > > > > > > > > > uverbs_destroy_ufile_hw() > > > > > mutex_lock() > > > > > [..] > > > > > mmput() > > > > > exit_mmap() > > > > > remove_vma() > > > > > fput(); > > > > > file_operations->release() > > > > > > > > I think this is wrong, and I'm pretty sure it's an example of why > > > > the final __fput() call is moved out of line. > > > > > > Yes, I think so too, all I can say is this *used* to happen, as we > > > have special code avoiding it, which is the code that is messing up > > > Ira's lifetime model. > > > > > > Ira, you could try unraveling the special locking, that solves your > > > lifetime issues? > > > > Yes I will try to prove this out... But I'm still not sure this fully solves > > the problem. > > > > This only ensures that the process which has the RDMA context (RDMA FD) is safe > > with regard to hanging the close for the "data file FD" (the file which has > > pinned pages) in that _same_ process. But what about the scenario. > > > > Process A has the RDMA context FD and data file FD (with lease) open. > > > > Process A uses SCM_RIGHTS to pass the RDMA context FD to Process B. > > Passing the RDMA context dependent on a file layout lease to another > process that doesn't have a file layout lease or a reference to the > original lease should be considered a violation of the layout lease. > Process B does not have an active layout lease, and so by the rules > of layout leases, it is not allowed to pin the layout of the file. > I don't disagree with the semantics of this. I just don't know how to enforce it. > > Process A attempts to exit (hangs because data file FD is pinned). > > > > Admin kills process A. kill works because we have allowed for it... > > > > Process B _still_ has the RDMA context FD open _and_ therefore still holds the > > file pins. > > > > Truncation still fails. > > > > Admin does not know which process is holding the pin. > > > > What am I missing? > > Application does not hold the correct file layout lease references. > Passing the fd via SCM_RIGHTS to a process without a layout lease > is equivalent to not using layout leases in the first place. Ok, So If I understand you correctly you would support a failure of SCM_RIGHTS in this case? I'm ok with that but not sure how to implement it right now. To that end, I would like to simplify this slightly because I'm not convinced that SCM_RIGHTS is a problem we need to solve right now. ie I don't know of a user who wants to do this.

Right now duplication via SCM_RIGHTS could fail if _any_ file pins (and by definition leases) exist underneath the "RDMA FD" (or other direct access FD, like XDP etc) being duplicated.