Base ==== This series is based on linux-next/akpm. Additionally, this series depends on Peter Xu's series to allow disabling huge pmd sharing. [1] https://lore.kernel.org/patchwork/cover/1382204/ Changelog ========= v6->v7: - Based upon discussion, switched back to the VM_* flags approach which was used in v5, instead of implementing this as an API feature. Switched to using a high bit (instead of brokenly conflicting with VM_LOCKED), which implies introducing CONFIG_HAVE_ARCH_USERFAULTFD_MINOR and selecting it only on 64-bit architectures (x86_64 and arm64 for now). v5->v6: - Fixed the condition guarding a second case where we unlock_page() in hugetlb_mcopy_atomic_pte(). - Significantly refactored how minor registration works. Because there are no VM_* flags available to use, it has to be a userfaultfd API feature, rather than a registration mode. This has a few knock on consequences worth calling out: - userfaultfd_minor() can no longer be inline, because we have to inspect the userfaultfd_ctx, which is only defined in fs/userfaultfd.c. This means slightly more overhead (1 function call) on all hugetlbfs minor faults. - vma_can_userfault() no longer changes. It seems valid to me to create an FD with the minor fault feature enabled, and then register e.g. some non-hugetlbfs region in MISSING mode, fully expecting to not get any minor faults for it, alongside some other region which you *do* want minor faults for. So, at registration time, either should be accepted. - Since I'm no longer adding a new registration mode, I'm no longer introducing __VM_UFFD_FLAGS or UFFD_API_REGISTER_MODES, and all the related cleanups have been reverted. v4->v5: - Typo fix in the documentation update. - Removed comment in vma_can_userfault. The same information is better covered in the documentation update, so the comment is unnecessary (and slightly confusing as written). - Reworded comment for MCOPY_ATOMIC_CONTINUE mode. - For non-shared CONTINUE, only make the PTE(s) non-writable, don't change flags on the VMA. - In hugetlb_mcopy_atomic_pte, always unlock the page in MCOPY_ATOMIC_CONTINUE, even if we don't have VM_SHARED. - In hugetlb_mcopy_atomic_pte, introduce "bool is_continue" to make that kind of mode check more terse. - Merged two nested if()s into a single expression in __mcopy_atomic_hugetlb. - Moved "return -EINVAL if MCOPY_CONTINUE isn't supported for this vma type" up one level, into __mcopy_atomic. - Rebased onto linux-next/akpm, instead of the latest 5.11 RC. Resolved conflicts with Mike's recent hugetlb changes. v3->v4: - Relaxed restriction for minor registration to allow any hugetlb VMAs, not just those with VM_SHARED. Fixed setting VM_WRITE flag in a CONTINUE ioctl for non-VM_SHARED VMAs. - Reordered if() branches in hugetlb_mcopy_atomic_pte, so the conditions are simpler and easier to read. - Reverted most of the mfill_atomic_pte change (the anon / shmem path). Just return -EINVAL for CONTINUE, and set zeropage = (mode == MCOPY_ATOMIC_ZEROPAGE), so we can keep the delta small. - Split out adding #ifdef CONFIG_USERFAULTFD to a separate patch (instead of lumping it together with adding UFFDIO_CONTINUE). - Fixed signature of hugetlb_mcopy_atomic_pte for !CONFIG_HUGETLB_PAGE (signature must be the same in either case). - Rebased onto a newer version of Peter's patches to disable huge PMD sharing. v2->v3: - Added #ifdef CONFIG_USERFAULTFD around hugetlb helper functions, to fix build errors when building without CONFIG_USERFAULTFD set. v1->v2: - Fixed a bug in the hugetlb_mcopy_atomic_pte retry case. We now plumb in the enum mcopy_atomic_mode, so we can differentiate between the three cases this function needs to handle: 1) We're doing a COPY op, and need to allocate a page, add to cache, etc. 2) We're doing a COPY op, but allocation in this function failed previously; we're in the retry path. The page was allocated, but not e.g. added to page cache, so that still needs to be done. 3) We're doing a CONTINUE op, we need to look up an existing page instead of allocating a new one. - Rebased onto a newer version of Peter's patches to disable huge PMD sharing, which fixes syzbot complaints on some non-x86 architectures. - Moved __VM_UFFD_FLAGS into userfaultfd_k.h, so inline helpers can use it. - Renamed UFFD_FEATURE_MINOR_FAULT_HUGETLBFS to UFFD_FEATURE_MINOR_HUGETLBFS, for consistency with other existing feature flags. - Moved the userfaultfd_minor hook in hugetlb.c into the else block, so we don't have to explicitly check for !new_page. RFC->v1: - Rebased onto Peter Xu's patches for disabling huge PMD sharing for certain userfaultfd-registered areas. - Added commits which update documentation, and add a self test which exercises the new feature. - Fixed reporting CONTINUE as a supported ioctl even for non-MINOR ranges. Overview ======== This series adds a new userfaultfd feature, UFFD_FEATURE_MINOR_HUGETLBFS. When enabled (via the UFFDIO_API ioctl), this feature means that any hugetlbfs VMAs registered with UFFDIO_REGISTER_MODE_MISSING will *also* get events for "minor" faults. By "minor" fault, I mean the following situation: Let there exist two mappings (i.e., VMAs) to the same page(s) (shared memory). One of the mappings is registered with userfaultfd (in minor mode), and the other is not. Via the non-UFFD mapping, the underlying pages have already been allocated & filled with some contents. The UFFD mapping has not yet been faulted in; when it is touched for the first time, this results in what I'm calling a "minor" fault. As a concrete example, when working with hugetlbfs, we have huge_pte_none(), but find_lock_page() finds an existing page. We also add a new ioctl to resolve such faults: UFFDIO_CONTINUE. The idea is, userspace resolves the fault by either a) doing nothing if the contents are already correct, or b) updating the underlying contents using the second, non-UFFD mapping (via memcpy/memset or similar, or something fancier like RDMA, or etc...). In either case, userspace issues UFFDIO_CONTINUE to tell the kernel "I have ensured the page contents are correct, carry on setting up the mapping". Use Case ======== Consider the use case of VM live migration (e.g. under QEMU/KVM): 1. While a VM is still running, we copy the contents of its memory to a target machine. The pages are populated on the target by writing to the non-UFFD mapping, using the setup described above. The VM is still running (and therefore its memory is likely changing), so this may be repeated several times, until we decide the target is "up to date enough". 2. We pause the VM on the source, and start executing on the target machine. During this gap, the VM's user(s) will *see* a pause, so it is desirable to minimize this window. 3. Between the last time any page was copied from the source to the target, and when the VM was paused, the contents of that page may have changed - and therefore the copy we have on the target machine is out of date. Although we can keep track of which pages are out of date, for VMs with large amounts of memory, it is "slow" to transfer this information to the target machine. We want to resume execution before such a transfer would complete. 4. So, the guest begins executing on the target machine. The first time it touches its memory (via the UFFD-registered mapping), userspace wants to intercept this fault. Userspace checks whether or not the page is up to date, and if not, copies the updated page from the source machine, via the non-UFFD mapping. Finally, whether a copy was performed or not, userspace issues a UFFDIO_CONTINUE ioctl to tell the kernel "I have ensured the page contents are correct, carry on setting up the mapping". We don't have to do all of the final updates on-demand. The userfaultfd manager can, in the background, also copy over updated pages once it receives the map of which pages are up-to-date or not. Interaction with Existing APIs ============================== Because this is a feature, a registered VMA could potentially receive both missing and minor faults. I spent some time thinking through how the existing API interacts with the new feature: UFFDIO_CONTINUE cannot be used to resolve non-minor faults, as it does not allocate a new page. If UFFDIO_CONTINUE is used on a non-minor fault: - For non-shared memory or shmem, -EINVAL is returned. - For hugetlb, -EFAULT is returned. UFFDIO_COPY and UFFDIO_ZEROPAGE cannot be used to resolve minor faults. Without modifications, the existing codepath assumes a new page needs to be allocated. This is okay, since userspace must have a second non-UFFD-registered mapping anyway, thus there isn't much reason to want to use these in any case (just memcpy or memset or similar). - If UFFDIO_COPY is used on a minor fault, -EEXIST is returned. - If UFFDIO_ZEROPAGE is used on a minor fault, -EEXIST is returned (or -EINVAL in the case of hugetlb, as UFFDIO_ZEROPAGE is unsupported in any case). - UFFDIO_WRITEPROTECT simply doesn't work with shared memory, and returns -ENOENT in that case (regardless of the kind of fault). Future Work =========== Currently the patchset only supports hugetlbfs. There is no reason it can't work with shmem, but I expect hugetlbfs to be much more commonly used since we're talking about backing guest memory for VMs. I plan to implement shmem support in a follow-up patch series. Axel Rasmussen (6): userfaultfd: add minor fault registration mode userfaultfd: disable huge PMD sharing for MINOR registered VMAs userfaultfd: hugetlbfs: only compile UFFD helpers if config enabled userfaultfd: add UFFDIO_CONTINUE ioctl userfaultfd: update documentation to describe minor fault handling userfaultfd/selftests: add test exercising minor fault handling Documentation/admin-guide/mm/userfaultfd.rst | 107 +++++++----- arch/arm64/Kconfig | 1 + arch/x86/Kconfig | 1 + fs/proc/task_mmu.c | 3 + fs/userfaultfd.c | 146 +++++++++++++---- include/linux/hugetlb.h | 7 + include/linux/mm.h | 7 + include/linux/userfaultfd_k.h | 46 +++++- include/trace/events/mmflags.h | 9 +- include/uapi/linux/userfaultfd.h | 36 +++- init/Kconfig | 5 + mm/hugetlb.c | 75 +++++++-- mm/userfaultfd.c | 37 +++-- tools/testing/selftests/vm/userfaultfd.c | 164 ++++++++++++++++++- 14 files changed, 529 insertions(+), 115 deletions(-) -- 2.30.0.617.g56c4b15f3c-goog
