[ adding linux-rdma ] On Thu, Nov 30, 2017 at 10:17 AM, Michal Hocko <mhocko@xxxxxxxxxx> wrote: > > On Thu 30-11-17 10:03:26, Dan Williams wrote: > > On Thu, Nov 30, 2017 at 9:42 AM, Michal Hocko <mhocko@xxxxxxxxxx> wrote: > > > > > > 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 > > > > You're not misreading. See ib_umem_get() for example, it pins pages in > > response to the userspace library call ibv_reg_mr() (memory > > registration), and will not release those pages unless/until a call to > > ibv_dereg_mr() is made. > > Who and how many LRU pages can pin that way and how do you prevent nasty > users to DoS systems this way? I assume this is something the RDMA community has had to contend with? I'm not an RDMA person, I'm just here to fix dax. > I remember PeterZ wanted to address a similar issue by vmpin syscall > that would be a subject of a rlimit control. Sorry but I cannot find a > reference here https://lwn.net/Articles/600502/ > but if this is at g-u-p level without any accounting then > it smells quite broken to me. It's certainly broken with respect to filesystem-dax and if there is other breakage we should get it all on the table.