Hello everyone, This is the latest userfaultfd patchset against mm-v4.1-rc3 2015-05-14-10:04. The postcopy live migration feature on the qemu side is mostly ready to be merged and it entirely depends on the userfaultfd syscall to be merged as well. So it'd be great if this patchset could be reviewed for merging in -mm. Userfaults allow to implement on demand paging from userland and more generally they allow userland to more efficiently take control of the behavior of page faults than what was available before (PROT_NONE + SIGSEGV trap). The use cases are: 1) KVM postcopy live migration (one form of cloud memory externalization). KVM postcopy live migration is the primary driver of this work: http://blog.zhaw.ch/icclab/setting-up-post-copy-live-migration-in-openstack/ http://lists.gnu.org/archive/html/qemu-devel/2015-02/msg04873.html 2) postcopy live migration of binaries inside linux containers: http://thread.gmane.org/gmane.linux.kernel.mm/132662 3) KVM postcopy live snapshotting (allowing to limit/throttle the memory usage, unlike fork would, plus the avoidance of fork overhead in the first place). While the wrprotect tracking is not implemented yet, the syscall API is already contemplating the wrprotect fault tracking and it's generic enough to allow its later implementation in a backwards compatible fashion. 4) KVM userfaults on shared memory. The UFFDIO_COPY lowlevel method should be extended to work also on tmpfs and then the uffdio_register.ioctls will notify userland that UFFDIO_COPY is available even when the registered virtual memory range is tmpfs backed. 5) alternate mechanism to notify web browsers or apps on embedded devices that volatile pages have been reclaimed. This basically avoids the need to run a syscall before the app can access with the CPU the virtual regions marked volatile. This depends on point 4) to be fulfilled first, as volatile pages happily apply to tmpfs. Even though there wasn't a real use case requesting it yet, it also allows to implement distributed shared memory in a way that readonly shared mappings can exist simultaneously in different hosts and they can be become exclusive at the first wrprotect fault. The development version can also be cloned here: git clone --reference linux -b userfault git://git.kernel.org/pub/scm/linux/kernel/git/andrea/aa.git Slides from LSF-MM summit (but beware that they're not uptodate): https://www.kernel.org/pub/linux/kernel/people/andrea/userfaultfd/userfaultfd-LSFMM-2015.pdf Comments welcome. Thanks, Andrea Changelog of the major changes since the last RFC v3: o The API has been slightly modified to avoid having to introduce a second revision of the API, in order to support the non cooperative usage. o Various mixed fixes thanks to the feedback from Dave Hansen and David Gilbert. The most notable one is the use of mm_users instead of mm_count to pin the mm to avoid crashes that assumed the vma still existed (in the userfaultfd_release method and in the various ioctl). exit_mmap doesn't even set mm->mmap to NULL, so unless I introduce a userfaultfd_exit to call in mmput, I have to pin the mm_users to be safe. This is a visible change mainly for the non-cooperative usage. o userfaults are waken immediately even if they're not been "read" yet, this can lead to POLLIN false positives (so I only allow poll if the fd is open in nonblocking mode to be sure it won't hang). http://git.kernel.org/cgit/linux/kernel/git/andrea/aa.git/commit/?h=userfault&id=f222d9de0a5302dc8ac62d6fab53a84251098751 o optimize read to return entries in O(1) and poll which was already O(1) becomes lockless. This required to split the waitqueue in two, one for pending faults and one for non pending faults, and the faults are refiled across the two waitqueues when they're read. Both waitqueues are protected by a single lock to be simpler and faster at runtime (the fault_pending_wqh one). http://git.kernel.org/cgit/linux/kernel/git/andrea/aa.git/commit/?h=userfault&id=9aa033ed43a1134c2223dac8c5d9e02e0100fca1 o Allocate the ctx with kmem_cache_alloc. http://git.kernel.org/cgit/linux/kernel/git/andrea/aa.git/commit/?h=userfault&id=f5a8db16d2876eed8906a4d36f1d0e06ca5490f6 o Originally qemu had two bitflags for each page and kept 3 states (of the 4 possible with two bits) for each page in order to deal with the races that can happen if one thread is reading the userfaults and another thread is calling the UFFDIO_COPY ioctl in the background. This patch solves all races in the kernel so the two bits per page can be dropped from qemu codebase. I started documenting the races that can materialize by using 2 threads (instead of running the workload single threaded with a single poll event loop) and how userland had to solve them until I decided it was simpler to fix the race in the kernel by running an ad-hoc pagetable walk inside the wait_event()-kind-of-section. This simplified qemu significantly (hundreds line of code involving a mutex have been deleted and that mutex disappeared as well) and it doesn't make the kernel much more complicated. http://git.kernel.org/cgit/linux/kernel/git/andrea/aa.git/commit/?h=userfault&id=41efeae4e93f0296436f2a9fc6b28b6b0158512a After this patch the only reason to call UFFDIO_WAKE is to handle the userfaults in batches in combination with the DONT_WAKE flag of UFFDIO_COPY. o I removed the read recursion from mcopy_atomic. This avoids to depend on the write-starvation behavior of rwsem to be safe. After this change the rwsem is free to stop any further down_read if there's a down_write waiting on the lock. http://git.kernel.org/cgit/linux/kernel/git/andrea/aa.git/commit/?h=userfault&id=b1e3a08acc9e3f6c2614e89fc3b8e338daa58e18 o Extendeded the Documentation userfaultfd.txt file to explain how QEMU/KVM uses userfaultfd to implement postcopy live migration. http://git.kernel.org/cgit/linux/kernel/git/andrea/aa.git/commit/?h=userfault&id=016f9523b7b2238851533736e84452cb00b2ddcd Andrea Arcangeli (22): userfaultfd: linux/Documentation/vm/userfaultfd.txt userfaultfd: waitqueue: add nr wake parameter to __wake_up_locked_key userfaultfd: uAPI userfaultfd: linux/userfaultfd_k.h userfaultfd: add vm_userfaultfd_ctx to the vm_area_struct userfaultfd: add VM_UFFD_MISSING and VM_UFFD_WP userfaultfd: call handle_userfault() for userfaultfd_missing() faults userfaultfd: teach vma_merge to merge across vma->vm_userfaultfd_ctx userfaultfd: prevent khugepaged to merge if userfaultfd is armed userfaultfd: add new syscall to provide memory externalization userfaultfd: Rename uffd_api.bits into .features fixup userfaultfd: change the read API to return a uffd_msg userfaultfd: wake pending userfaults userfaultfd: optimize read() and poll() to be O(1) userfaultfd: allocate the userfaultfd_ctx cacheline aligned userfaultfd: solve the race between UFFDIO_COPY|ZEROPAGE and read userfaultfd: buildsystem activation userfaultfd: activate syscall userfaultfd: UFFDIO_COPY|UFFDIO_ZEROPAGE uAPI userfaultfd: mcopy_atomic|mfill_zeropage: UFFDIO_COPY|UFFDIO_ZEROPAGE preparation userfaultfd: avoid mmap_sem read recursion in mcopy_atomic userfaultfd: UFFDIO_COPY and UFFDIO_ZEROPAGE Pavel Emelyanov (1): userfaultfd: Rename uffd_api.bits into .features Documentation/ioctl/ioctl-number.txt | 1 + Documentation/vm/userfaultfd.txt | 142 ++++ arch/powerpc/include/asm/systbl.h | 1 + arch/powerpc/include/uapi/asm/unistd.h | 1 + arch/x86/syscalls/syscall_32.tbl | 1 + arch/x86/syscalls/syscall_64.tbl | 1 + fs/Makefile | 1 + fs/proc/task_mmu.c | 2 + fs/userfaultfd.c | 1236 ++++++++++++++++++++++++++++++++ include/linux/mm.h | 4 +- include/linux/mm_types.h | 11 + include/linux/syscalls.h | 1 + include/linux/userfaultfd_k.h | 85 +++ include/linux/wait.h | 5 +- include/uapi/linux/Kbuild | 1 + include/uapi/linux/userfaultfd.h | 161 +++++ init/Kconfig | 11 + kernel/fork.c | 3 +- kernel/sched/wait.c | 7 +- kernel/sys_ni.c | 1 + mm/Makefile | 1 + mm/huge_memory.c | 75 +- mm/madvise.c | 3 +- mm/memory.c | 16 + mm/mempolicy.c | 4 +- mm/mlock.c | 3 +- mm/mmap.c | 40 +- mm/mprotect.c | 3 +- mm/userfaultfd.c | 309 ++++++++ net/sunrpc/sched.c | 2 +- 30 files changed, 2082 insertions(+), 50 deletions(-) create mode 100644 Documentation/vm/userfaultfd.txt create mode 100644 fs/userfaultfd.c create mode 100644 include/linux/userfaultfd_k.h create mode 100644 include/uapi/linux/userfaultfd.h create mode 100644 mm/userfaultfd.c Credits: partially funded by the Orbit EU project. -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
