Hello, Since userfaultfd remap functionality was first proposed by Andrea Arcangeli [1], a new use case has been demonstrated for removing pages from the userfaultfd registered region. FluidMem [2] is a system for expanding or limiting the resident size of a VM using a remote key-value store as backing storage instead of swap space. It runs on the hypervisor and uses userfaultfd to manage the memory regions malloc'd by qemu. Since FluidMem maintains a constant resident size using an LRU list, it must evict pages to the remote key-value store to make room for pages that were just faulted in. This requires UFFDIO_REMAP to remove pages from the uncooperative userspace page fault handler. The VM shadow page tables must be kept in sync after a remapping, so mmu_notifier_invalidate_range_(start/end) calls are made as necessary. FluiMem enables page fault latencies to a remote key-value store that are as fast as swap backed by DRAM (/dev/pmem0) and 77% faster than swap with a SSD drive. pmbench [3] was used to measure page fault latencies with a 4 GB working set size, within a VM using 1 GB DRAM (20% local): FluidMem (RAMCloud): 24.87 microseconds Swap (pmem DRAM): 26.34 microseconds Swap (NVMe over Fabrics): 41.73 microseconds Swap (SSD): 106.56 microseconds For real applications FluidMem has an additional benefit of allowing unused kernel pages to be removed from DRAM and stored in backend storage, making room for additional application pages to be kept in local DRAM. The useful memory capacity for the VM is increased. The main complexity of this code is found in rmap, where it overwrites the page->index when it moves the page to a different vma with different vma->vm_pgoff. Overwriting page->index requires the rmap change and it's only possible when the page_mapcount is 1. Changes since [1]: - Changed the direction supported by UFFDIO_REMAP to the OUT direction needed by FluidMem. The IN direction is not necessary, as UFFDIO_COPY should be used instead because it doesn't require a TLB flush. - Code has been kept up-to-date by Andrea in branch userfault from [4]. [1] https://lkml.org/lkml/2015/3/5/576 [2] Caldwell, Blake, Youngbin Im, Sangtae Ha, Richard Han, and Eric Keller. "FluidMem: Memory as a Service for the Datacenter." arXiv preprint arXiv:1707.07780 (2017). https://github.com/blakecaldwell/fluidmem [3] Yang, Jisoo, and Julian Seymour. "Pmbench: A Micro-Benchmark for Profiling Paging Performance on a System with Low-Latency SSDs." Information Technology-New Generations. Springer, Cham, 2018. 627-633. https://bitbucket.org/jisooy/pmbench/src [4] https://git.kernel.org/pub/scm/linux/kernel/git/andrea/aa.git Andrea Arcangeli (3): userfaultfd: UFFDIO_REMAP: rmap preparation userfaultfd: UFFDIO_REMAP uABI userfaultfd: UFFDIO_REMAP Blake Caldwell (1): userfaultfd: change the direction for UFFDIO_REMAP to out Documentation/admin-guide/mm/userfaultfd.rst | 10 + fs/userfaultfd.c | 49 +++ include/linux/userfaultfd_k.h | 17 + include/uapi/linux/userfaultfd.h | 25 +- mm/huge_memory.c | 117 ++++++ mm/khugepaged.c | 3 + mm/rmap.c | 13 + mm/userfaultfd.c | 536 +++++++++++++++++++++++++++ 8 files changed, 769 insertions(+), 1 deletion(-) -- 1.8.3.1
