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

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Message-ID: <20190812130039.GD24457@ziepe.ca>
Date:   Mon, 12 Aug 2019 10:00:40 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     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 Fri, Aug 09, 2019 at 03:58:30PM -0700, ira.weiny@...el.com wrote:
> From: Ira Weiny <ira.weiny@...el.com>
> 
> In order for MRs to be tracked against the open verbs context the ufile
> needs to have a pointer to hand to the GUP code.
> 
> No references need to be taken as this should be valid for the lifetime
> of the context.
> 
> Signed-off-by: Ira Weiny <ira.weiny@...el.com>
>  drivers/infiniband/core/uverbs.h      | 1 +
>  drivers/infiniband/core/uverbs_main.c | 1 +
>  2 files changed, 2 insertions(+)
> 
> diff --git a/drivers/infiniband/core/uverbs.h b/drivers/infiniband/core/uverbs.h
> index 1e5aeb39f774..e802ba8c67d6 100644
> +++ b/drivers/infiniband/core/uverbs.h
> @@ -163,6 +163,7 @@ struct ib_uverbs_file {
>  	struct page *disassociate_page;
>  
>  	struct xarray		idr;
> +	struct file             *sys_file; /* backpointer to system file object */
>  };

The 'struct file' has a lifetime strictly shorter than the
ib_uverbs_file, which is kref'd on its own lifetime. Having a back
pointer like this is confouding as it will be invalid for some of the
lifetime of the struct.

Jason