linux-ext4 - Re: [RFC PATCH v2 00/19] RDMA/FS DAX truncate proposal V1,000,002 ;-)

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Date:   Tue, 20 Aug 2019 08:55:15 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Dave Chinner <david@...morbit.com>
Cc:     Jan Kara <jack@...e.cz>, Ira Weiny <ira.weiny@...el.com>,
        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 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?

Jason