On Tue 15-08-17 16:50:55, Dan Williams wrote: > On Tue, Aug 15, 2017 at 1:37 AM, Jan Kara <jack@xxxxxxx> wrote: > > On Mon 14-08-17 09:14:42, Dan Williams wrote: > >> On Mon, Aug 14, 2017 at 5:40 AM, Jan Kara <jack@xxxxxxx> wrote: > >> > On Sun 13-08-17 13:31:45, Dan Williams wrote: > >> >> On Sun, Aug 13, 2017 at 2:24 AM, Christoph Hellwig <hch@xxxxxx> wrote: > >> >> > Thay being said I think we absolutely should support RDMA memory > >> >> > registrations for DAX mappings. I'm just not sure how S_IOMAP_IMMUTABLE > >> >> > helps with that. We'll want a MAP_SYNC | MAP_POPULATE to make sure > >> >> > all the blocks are polulated and all ptes are set up. Second we need > >> >> > to make sure get_user_page works, which for now means we'll need a > >> >> > struct page mapping for the region (which will be really annoying > >> >> > for PCIe mappings, like the upcoming NVMe persistent memory region), > >> >> > and we need to gurantee that the extent mapping won't change while > >> >> > the get_user_pages holds the pages inside it. I think that is true > >> >> > due to side effects even with the current DAX code, but we'll need to > >> >> > make it explicit. And maybe that's where we need to converge - > >> >> > "sealing" the extent map makes sense as such a temporary measure > >> >> > that is not persisted on disk, which automatically gets released > >> >> > when the holding process exits, because we sort of already do this > >> >> > implicitly. It might also make sense to have explicitl breakable > >> >> > seals similar to what I do for the pNFS blocks kernel server, as > >> >> > any userspace RDMA file server would also need those semantics. > >> >> > >> >> Ok, how about a MAP_DIRECT flag that arranges for faults to that range to: > >> >> > >> >> 1/ only succeed if the fault can be satisfied without page cache > >> >> > >> >> 2/ only install a pte for the fault if it can do so without > >> >> triggering block map updates > >> >> > >> >> So, I think it would still end up setting an inode flag to make > >> >> xfs_bmapi_write() fail while any process has a MAP_DIRECT mapping > >> >> active. However, it would not record that state in the on-disk > >> >> metadata and it would automatically clear at munmap time. That should > >> >> be enough to support the host-persistent-memory, and > >> >> NVMe-persistent-memory use cases (provided we have struct page for > >> >> NVMe). Although, we need more safety infrastructure in the NVMe case > >> >> where we would need to software manage I/O coherence. > >> > > >> > Hum, this proposal (and the problems you are trying to deal with) seem very > >> > similar to Peter Zijlstra's mpin() proposal from 2014 [1], just moved to > >> > the DAX area (and so additionally complicated by the fact that filesystems > >> > now have to care). The patch set was not merged due to lack of interest I > >> > think but it looked sensible and the proposed API would make sense for more > >> > stuff than just DAX so maybe it would be better than MAP_DIRECT flag? > >> > >> Interesting, but I'm not sure I see the correlation. mm_mpin() makes a > >> "no-fault" guarantee and fixes the accounting of locked System RAM. > >> MAP_DIRECT still allows faults, and DAX mappings don't consume System > >> RAM so the accounting problem is not there for DAX. mm_pin() also does > >> not appear to have a relationship to a file backed memory like mmap > >> allows. > > > > So the accounting part is probably non-interesting for DAX purposes and I > > agree there are other differences as well. But mm_mpin() prevented page > > migrations which is parallel to your requirement of "offset->block mapping > > is permanent". Furthermore mm_mpin() work was there for RDMA so that it > > has saner interface to pin pages than get_user_pages() and you mention RDMA > > and similar technologies as a usecase for your work for similar reasons. > > So my thought was that possibly we should have the same API for pinning > > "storage" for RDMA transfers regardless of whether the backing is page > > cache or pmem and the API should be usable for in-kernel users as well? > > mmap flag seems a bit clumsy in this regard so maybe a form of a separate > > syscall - be it mpin(start, len) or some other name - might be more > > suitable? > > Can you say about more about why an mmap flag for this feels awkward > to you? I think there's symmetry between O_SYNC / O_DIRECT setting up > synchronous / page-cache-bypass file descriptors and MAP_SYNC / > MAP_DIRECT setting up synchronous and page-cache bypass mappings. So my thinking was, that for in-kernel users it might be a bit more difficult to use mmap flag directly as they generally won't need to setup the mapping. But that can be certainly dealt with by proper helpers for in-kernel users. > "Pinning" also feels like the wrong mechanism when you consider > hardware is moving toward eliminating the pinning requirement over > time. SVM "Shared Virtual Memory" hardware will just operate on cpu > virtual addresses directly and generate typical faults. On such > hardware MAP_DIRECT would be a nop relative to MAP_SYNC, so you > wouldn't want your application to be stuck with the legacy concept > that pages need to be explicitly "pinned". OK, makes sense. Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
