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Subject[LSF/MM TOPIC] Discuss least bad options for resolving longterm-GUP usage by RDMA

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

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