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
