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

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Date:   Wed, 14 Aug 2019 09:23:08 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Ira Weiny <ira.weiny@...el.com>
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 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.

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