linux-ext4 - Re: [RFC PATCH v2 16/19] RDMA/uverbs: Add back pointer to system file object

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Date:   Wed, 4 Sep 2019 15:25:50 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>,
        Theodore Ts'o <tytso@....edu>,
        John Hubbard <jhubbard@...dia.com>,
        Michal Hocko <mhocko@...e.com>,
        Dave Chinner <david@...morbit.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 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?

Ira

> 
> 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