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

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Date:   Tue, 13 Aug 2019 15:00:22 -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 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.
 
> > This is why having a back pointer like this is so ugly, it creates a
> > reference counting cycle
> 
> Yep...  I worked through this...  and it was giving me fits...
> 
> Anyway, the struct file is the only object in the core which was reasonable to
> store this information in since that is what is passed around to other
> processes...

It could be passed down in the uattr_bundle, once you are in file operations
handle the file is guarenteed to exist, and we've now arranged things
so the uattr_bundle flows into the umem, as umems can only be
established under a system call.

Jason