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

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Message-ID: <20190821181343.GH8653@ziepe.ca>
Date:   Wed, 21 Aug 2019 15:13:43 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Ira Weiny <ira.weiny@...el.com>
Cc:     Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Matthew Wilcox <willy@...radead.org>,
        Theodore Ts'o <tytso@....edu>,
        John Hubbard <jhubbard@...dia.com>,
        Michal Hocko <mhocko@...e.com>, linux-xfs@...r.kernel.org,
        linux-rdma@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-nvdimm@...ts.01.org,
        linux-ext4@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC PATCH v2 00/19] RDMA/FS DAX truncate proposal V1,000,002 ;-)

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.

Oh, I didn't think we were talking about that. Hanging the close of
the datafile fd contingent on some other FD's closure is a recipe for
deadlock..

IMHO the pin refcnt is held by the driver char dev FD, that is the
object you need to make it visible against.

Why not just have a single table someplace of all the layout leases
with the file they are held on and the FD/socket/etc that is holding
the pin? Make it independent of processes and FDs?

Jason