On Thu 30-11-17 08:39:51, Dan Williams wrote: > On Thu, Nov 30, 2017 at 1:53 AM, Michal Hocko <mhocko@xxxxxxxxxx> wrote: > > On Wed 29-11-17 10:05:35, Dan Williams wrote: > >> Until there is a solution to the dma-to-dax vs truncate problem it is > >> not safe to allow long standing memory registrations against > >> filesytem-dax vmas. Device-dax vmas do not have this problem and are > >> explicitly allowed. > >> > >> This is temporary until a "memory registration with layout-lease" > >> mechanism can be implemented for the affected sub-systems (RDMA and > >> V4L2). > > > > One thing is not clear to me. Who is allowed to pin pages for ever? > > Is it possible to pin LRU pages that way as well? If yes then there > > absolutely has to be a limit for that. Sorry I could have studied the > > code much more but from a quick glance it seems to me that this is not > > limited to dax (or non-LRU in general) pages. > > I would turn this question around. "who can not tolerate a page being > pinned forever?". Any struct page on the movable zone or anything that is living on the LRU list because such a memory is unreclaimable. > In the case of filesytem-dax a page is > one-in-the-same object as a filesystem-block, and a filesystem expects > that its operations will not be blocked indefinitely. LRU pages can > continue to be pinned indefinitely because operations can continue > around the pinned page, i.e. every agent, save for the dma agent, > drops their reference to the page and its tolerable that the final > put_page() never arrives. I do not understand. Are you saying that a user triggered IO can pin LRU pages indefinitely. This would be _really_ wrong. It would be basically an mlock without any limit. So I must be misreading you here > As far as I can tell it's only filesystems > and dax that have this collision of wanting to revoke dma access to a > page combined with not being able to wait indefinitely for dma to > quiesce. -- Michal Hocko SUSE Labs