linux-kernel - Re: [LSF/MM TOPIC] Discuss least bad options for resolving longterm-GUP usage by RDMA

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Date:   Tue, 5 Feb 2019 10:01:20 -0800
From:   Ira Weiny <ira.weiny@...el.com>
To:     lsf-pc@...ts.linux-foundation.org, linux-rdma@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Cc:     John Hubbard <jhubbard@...dia.com>, Jan Kara <jack@...e.cz>,
        Jerome Glisse <jglisse@...hat.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Matthew Wilcox <willy@...radead.org>,
        Dave Chinner <david@...morbit.com>,
        Doug Ledford <dledford@...hat.com>,
        Michal Hocko <mhocko@...nel.org>,
        Jason Gunthorpe <jgg@...pe.ca>
Subject: Re: [LSF/MM TOPIC] Discuss least bad options for resolving
 longterm-GUP usage by RDMA

I had an old invalid address for Jason Gunthorpe in my address book...  

Correcting his email in the thread.

On Tue, Feb 05, 2019 at 09:50:59AM -0800, 'Ira Weiny' wrote:
> 
> The problem: Once we have pages marked as GUP-pinned how should various
> subsystems work with those markings.
> 
> The current work for John Hubbards proposed solutions (part 1 and 2) is
> progressing.[1]  But the final part (3) of his solution is also going to take
> some work.
> 
> In Johns presentation he lists 3 alternatives for gup-pinned pages:
> 
> 1) Hold off try_to_unmap
> 2) Allow writeback while pinned (via bounce buffers)
> 	[Note this will not work for DAX]
> 3) Use a "revocable reservation" (or lease) on those pages
> 4) Pin the blocks as busy in the FS allocator
> 
> The problem with lease's on pages used by RDMA is that the references to
> these pages is not local to the machine.  Once the user has been given access
> to the page they, through the use of a remote tokens, give a reference to that
> page to remote nodes.  This is the core essence of RDMA, and like it or not,
> something which is increasingly used by major Linux users.
> 
> Therefore we need to discuss the extent by which leases are appropriate and
> what happens should a lease be revoked which a user does not respond to.
> 
> As John Hubbard put it:
> 
> "Other filesystem features that need to replace the page with a new one can
> be inhibited for pages that are GUP-pinned. This will, however, alter and
> limit some of those filesystem features. The only fix for that would be to
> require GUP users monitor and respond to CPU page table updates. Subsystems
> such as ODP and HMM do this, for example. This aspect of the problem is
> still under discussion."
> 
> 	-- John Hubbard[2]
> 
> The following people have been involved in previous conversations and would be key to
> the face to face discussion.
> 
> John Hubbard
> Jan Kara
> Dave Chinner
> Michal Hocko
> Dan Williams
> Matthew Wilcox
> Jason Gunthorpe
> 
> Thank you,
> Ira Weiny
> 
> [1] https://linuxplumbersconf.org/event/2/contributions/126/attachments/136/168/LPC_2018_gup_dma.pdf
> [2] https://lkml.org/lkml/2019/2/4/7
>