Add an ioctl to toggle async mode at runtime without re-registering the userfaultfd. This allows a VMM to switch between sync and async RWP modes on-the-fly -- for example, starting in async mode for working set scanning, then switching to sync mode to intercept faults during page eviction.
UFFDIO_SET_MODE takes an enable/disable bitmask of UFFD_FEATURE_* flags. Only UFFD_FEATURE_RWP_ASYNC is toggleable today; the ioctl rejects any other bit with -EINVAL. Enabling RWP_ASYNC also requires RWP to have been negotiated at UFFDIO_API time, mirroring the UFFDIO_API invariant. Fault-path readers of ctx->features run under mmap_read_lock or a per-VMA lock; the RMW takes mmap_write_lock and calls vma_start_write() on every UFFD-armed VMA, so those readers are fully excluded. userfaultfd_show_fdinfo(), however, reads ctx->features without any lock, so the RMW is written as a single WRITE_ONCE and fdinfo reads it with READ_ONCE. That keeps the lockless observer from seeing a mid-RMW intermediate and removes the audit burden when new toggleable bits are added later. When switching to async, pending sync waiters are woken so they retry and auto-resolve under the new mode. Signed-off-by: Kiryl Shutsemau (Meta) <[email protected]> Assisted-by: Claude:claude-opus-4-6 --- fs/userfaultfd.c | 130 +++++++++++++++++++++++++------ include/uapi/linux/userfaultfd.h | 14 ++++ 2 files changed, 120 insertions(+), 24 deletions(-) diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c index 4a701ac830f4..83e759054464 100644 --- a/fs/userfaultfd.c +++ b/fs/userfaultfd.c @@ -1871,6 +1871,107 @@ static int userfaultfd_rwprotect(struct userfaultfd_ctx *ctx, return ret; } +/* Subset of UFFD_API_FEATURES actually supported by this kernel/arch */ +static __u64 uffd_api_available_features(void) +{ + __u64 f = UFFD_API_FEATURES; + + if (!IS_ENABLED(CONFIG_HAVE_ARCH_USERFAULTFD_MINOR)) + f &= ~(UFFD_FEATURE_MINOR_HUGETLBFS | UFFD_FEATURE_MINOR_SHMEM); + if (!pgtable_supports_uffd()) + f &= ~UFFD_FEATURE_PAGEFAULT_FLAG_WP; + if (!uffd_supports_wp_marker()) + f &= ~(UFFD_FEATURE_WP_HUGETLBFS_SHMEM | + UFFD_FEATURE_WP_UNPOPULATED | + UFFD_FEATURE_WP_ASYNC); + /* + * RWP needs both PROT_NONE support and the uffd PTE bit. The + * VM_UFFD_RWP check covers compile-time unavailability; the + * pgtable_supports_uffd() check covers runtime (e.g. riscv + * without the SVRSW60T59B extension) where the PTE bit is declared + * but not actually usable. + */ + if (VM_UFFD_RWP == VM_NONE || !pgtable_supports_uffd()) + f &= ~(UFFD_FEATURE_RWP | UFFD_FEATURE_RWP_ASYNC); + return f; +} + +/* Async features that can be toggled at runtime via UFFDIO_SET_MODE */ +#define UFFD_FEATURE_TOGGLEABLE UFFD_FEATURE_RWP_ASYNC + +static int userfaultfd_set_mode(struct userfaultfd_ctx *ctx, + unsigned long arg) +{ + struct uffdio_set_mode mode; + struct mm_struct *mm = ctx->mm; + + if (copy_from_user(&mode, (void __user *)arg, sizeof(mode))) + return -EFAULT; + + /* enable and disable must not overlap */ + if (mode.enable & mode.disable) + return -EINVAL; + + /* only toggleable features that this kernel/arch actually supports */ + if ((mode.enable | mode.disable) & + ~(uffd_api_available_features() & UFFD_FEATURE_TOGGLEABLE)) + return -EINVAL; + + /* RWP_ASYNC can only be enabled on contexts that negotiated RWP */ + if ((mode.enable & UFFD_FEATURE_RWP_ASYNC) && + !(ctx->features & UFFD_FEATURE_RWP)) + return -EINVAL; + + if (!mmget_not_zero(mm)) + return -ESRCH; + + /* + * Drain in-flight faults before flipping features. mmap_write_lock() + * blocks new mmap_read_lock() callers, but per-VMA locked faults + * (lock_vma_under_rcu() + FAULT_FLAG_VMA_LOCK) that acquired before + * this point keep running. Calling vma_start_write() on each UFFD- + * armed VMA waits for those readers to drop, so no in-flight fault + * can observe the old features after mmap_write_unlock(). + */ + mmap_write_lock(mm); + { + struct vm_area_struct *vma; + VMA_ITERATOR(vmi, mm, 0); + + for_each_vma(vmi, vma) { + if (vma->vm_userfaultfd_ctx.ctx == ctx) + vma_start_write(vma); + } + } + /* + * Single WRITE_ONCE so the fdinfo lockless reader can't observe a + * mid-RMW intermediate value. Hot-path readers already serialise + * through the mmap lock + vma_start_write() drain above, so their + * load doesn't need an annotation. + */ + WRITE_ONCE(ctx->features, + (ctx->features | mode.enable) & ~mode.disable); + mmap_write_unlock(mm); + + /* + * If switching to async, wake threads blocked in handle_userfault(). + * They will retry the fault and auto-resolve under the new mode. + * len=0 means wake all pending faults on this context. + */ + if (mode.enable & UFFD_FEATURE_RWP_ASYNC) { + struct userfaultfd_wake_range range = { .len = 0 }; + + spin_lock_irq(&ctx->fault_pending_wqh.lock); + __wake_up_locked_key(&ctx->fault_pending_wqh, TASK_NORMAL, + &range); + __wake_up(&ctx->fault_wqh, TASK_NORMAL, 1, &range); + spin_unlock_irq(&ctx->fault_pending_wqh.lock); + } + + mmput(mm); + return 0; +} + static int userfaultfd_continue(struct userfaultfd_ctx *ctx, unsigned long arg) { __s64 ret; @@ -2109,29 +2210,7 @@ static int userfaultfd_api(struct userfaultfd_ctx *ctx, goto err_out; /* report all available features and ioctls to userland */ - uffdio_api.features = UFFD_API_FEATURES; -#ifndef CONFIG_HAVE_ARCH_USERFAULTFD_MINOR - uffdio_api.features &= - ~(UFFD_FEATURE_MINOR_HUGETLBFS | UFFD_FEATURE_MINOR_SHMEM); -#endif - if (!pgtable_supports_uffd()) - uffdio_api.features &= ~UFFD_FEATURE_PAGEFAULT_FLAG_WP; - - if (!uffd_supports_wp_marker()) { - uffdio_api.features &= ~UFFD_FEATURE_WP_HUGETLBFS_SHMEM; - uffdio_api.features &= ~UFFD_FEATURE_WP_UNPOPULATED; - uffdio_api.features &= ~UFFD_FEATURE_WP_ASYNC; - } - /* - * RWP needs both PROT_NONE support and the uffd-wp PTE bit. The - * VM_UFFD_RWP check covers compile-time unavailability; the - * pgtable_supports_uffd() check covers runtime (e.g. riscv - * without the SVRSW60T59B extension) where the PTE bit is declared - * but not actually usable. - */ - if (VM_UFFD_RWP == VM_NONE || !pgtable_supports_uffd()) - uffdio_api.features &= - ~(UFFD_FEATURE_RWP | UFFD_FEATURE_RWP_ASYNC); + uffdio_api.features = uffd_api_available_features(); ret = -EINVAL; if (features & ~uffdio_api.features) @@ -2201,6 +2280,9 @@ static long userfaultfd_ioctl(struct file *file, unsigned cmd, case UFFDIO_RWPROTECT: ret = userfaultfd_rwprotect(ctx, arg); break; + case UFFDIO_SET_MODE: + ret = userfaultfd_set_mode(ctx, arg); + break; } return ret; } @@ -2228,7 +2310,7 @@ static void userfaultfd_show_fdinfo(struct seq_file *m, struct file *f) * protocols: aa:... bb:... */ seq_printf(m, "pending:\t%lu\ntotal:\t%lu\nAPI:\t%Lx:%x:%Lx\n", - pending, total, UFFD_API, ctx->features, + pending, total, UFFD_API, READ_ONCE(ctx->features), UFFD_API_IOCTLS|UFFD_API_RANGE_IOCTLS); } #endif diff --git a/include/uapi/linux/userfaultfd.h b/include/uapi/linux/userfaultfd.h index c10f08f8a618..cea11aad6b54 100644 --- a/include/uapi/linux/userfaultfd.h +++ b/include/uapi/linux/userfaultfd.h @@ -49,6 +49,7 @@ #define UFFD_API_IOCTLS \ ((__u64)1 << _UFFDIO_REGISTER | \ (__u64)1 << _UFFDIO_UNREGISTER | \ + (__u64)1 << _UFFDIO_SET_MODE | \ (__u64)1 << _UFFDIO_API) #define UFFD_API_RANGE_IOCTLS \ ((__u64)1 << _UFFDIO_WAKE | \ @@ -85,6 +86,7 @@ #define _UFFDIO_CONTINUE (0x07) #define _UFFDIO_POISON (0x08) #define _UFFDIO_RWPROTECT (0x09) +#define _UFFDIO_SET_MODE (0x0A) #define _UFFDIO_API (0x3F) /* userfaultfd ioctl ids */ @@ -111,6 +113,8 @@ struct uffdio_poison) #define UFFDIO_RWPROTECT _IOWR(UFFDIO, _UFFDIO_RWPROTECT, \ struct uffdio_rwprotect) +#define UFFDIO_SET_MODE _IOW(UFFDIO, _UFFDIO_SET_MODE, \ + struct uffdio_set_mode) /* read() structure */ struct uffd_msg { @@ -406,6 +410,16 @@ struct uffdio_move { __s64 move; }; +struct uffdio_set_mode { + /* + * Toggle async mode for features at runtime. + * Supported: UFFD_FEATURE_RWP_ASYNC. + * Setting a bit in both enable and disable is invalid. + */ + __u64 enable; + __u64 disable; +}; + /* * Flags for the userfaultfd(2) system call itself. */ -- 2.51.2
