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

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Date:   Wed, 21 Aug 2019 11:57:03 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
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 03:13:43PM -0300, Jason Gunthorpe 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.
> 
> 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..

The discussion between Jan and Dave was concerning what happens when a user
calls

fd = open()
fnctl(...getlease...)
addr = mmap(fd...)
ib_reg_mr() <pin>
munmap(addr...)
close(fd)

Dave suggested:

"I'm of a mind to make the last close() on a file block if there's an
active layout lease to prevent processes from zombie-ing layout
leases like this. i.e. you can't close the fd until resources that
pin the lease have been released."

	-- Dave https://lkml.org/lkml/2019/8/16/994

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

I'm sorry but what do you mean by "driver char dev FD"?

> 
> 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?

If it is independent of processes how will we know which process is blocking
the truncate?  Using a global table is an interesting idea but I still believe
the users are going to want to track this to specific processes.  It's not
clear to me how that would be done with a global table.

I agree the XDP/socket case is bothersome...  I was thinking that somewhere the
fd of the socket could be hooked up in this case.  But taking a look at it
reveals that is not going to be easy.  And I assume XDP has the same issue WRT
SCM_RIGHTS and the ability to share the xdp context?

Ira

> 
> Jason