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Date:   Wed, 4 Sep 2019 15:25:50 -0700
From:   Ira Weiny <>
To:     Jason Gunthorpe <>
Cc:     Andrew Morton <>,
        Dan Williams <>,
        Matthew Wilcox <>, Jan Kara <>,
        Theodore Ts'o <>,
        John Hubbard <>,
        Michal Hocko <>,
        Dave Chinner <>,,,,,,,
Subject: Re: [RFC PATCH v2 16/19] RDMA/uverbs: Add back pointer to system
 file object

On Wed, Aug 14, 2019 at 09:23:08AM -0300, Jason Gunthorpe wrote:
> On Tue, Aug 13, 2019 at 01:38:59PM -0700, Ira Weiny wrote:
> > On Tue, Aug 13, 2019 at 03:00:22PM -0300, Jason Gunthorpe wrote:
> > > On Tue, Aug 13, 2019 at 10:41:42AM -0700, Ira Weiny wrote:
> > > 
> > > > And I was pretty sure uverbs_destroy_ufile_hw() would take care of (or ensure
> > > > that some other thread is) destroying all the MR's we have associated with this
> > > > FD.
> > > 
> > > fd's can't be revoked, so destroy_ufile_hw() can't touch them. It
> > > deletes any underlying HW resources, but the FD persists.
> > 
> > I misspoke.  I should have said associated with this "context".  And of course
> > uverbs_destroy_ufile_hw() does not touch the FD.  What I mean is that the
> > struct file which had file_pins hanging off of it would be getting its file
> > pins destroyed by uverbs_destroy_ufile_hw().  Therefore we don't need the FD
> > after uverbs_destroy_ufile_hw() is done.
> > 
> > But since it does not block it may be that the struct file is gone before the
> > MR is actually destroyed.  Which means I think the GUP code would blow up in
> > that case...  :-(
> Oh, yes, that is true, you also can't rely on the struct file living
> longer than the HW objects either, that isn't how the lifetime model
> works.

Reviewing all these old threads...  And this made me think.  While the HW
objects may out live the struct file.

They _are_ going away in a finite amount of time right?  It is not like they
could be held forever right?


> If GUP consumes the struct file it must allow the struct file to be
> deleted before the GUP pin is released.
> > The drivers could provide some generic object (in RDMA this could be the
> > uverbs_attr_bundle) which represents their "context".
> For RDMA the obvious context is the struct ib_mr *
> > But for the procfs interface, that context then needs to be associated with any
> > file which points to it...  For RDMA, or any other "FD based pin mechanism", it
> > would be up to the driver to "install" a procfs handler into any struct file
> > which _may_ point to this context.  (before _or_ after memory pins).
> Is this all just for debugging? Seems like a lot of complication just
> to print a string
> Generally, I think you'd be better to associate things with the
> mm_struct not some struct file... The whole design is simpler as GUP
> already has the mm_struct.
> Jason

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