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 >
