Add UFFDIO_SET_MODE ioctl to toggle UFFD_FEATURE_MINOR_ASYNC at runtime. Takes mmap_write_lock for serialization against all in-flight faults. On sync-to-async transition, wake threads blocked in handle_userfault() so they retry and auto-resolve.
Since ctx->features can now be modified concurrently, add userfaultfd_features() helper that wraps READ_ONCE() and convert all ctx->features reads to use it. Signed-off-by: Kiryl Shutsemau (Meta) <[email protected]> Assisted-by: Claude:claude-opus-4-6 --- fs/userfaultfd.c | 95 ++++++++++++++++++++++++++++---- include/uapi/linux/userfaultfd.h | 13 +++++ 2 files changed, 96 insertions(+), 12 deletions(-) diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c index 43064238fd8d..0edb33599491 100644 --- a/fs/userfaultfd.c +++ b/fs/userfaultfd.c @@ -79,24 +79,33 @@ struct userfaultfd_wake_range { /* internal indication that UFFD_API ioctl was successfully executed */ #define UFFD_FEATURE_INITIALIZED (1u << 31) +/* + * Read ctx->features with READ_ONCE() since UFFDIO_SET_MODE can + * modify it concurrently. + */ +static unsigned int userfaultfd_features(struct userfaultfd_ctx *ctx) +{ + return READ_ONCE(ctx->features); +} + static bool userfaultfd_is_initialized(struct userfaultfd_ctx *ctx) { - return ctx->features & UFFD_FEATURE_INITIALIZED; + return userfaultfd_features(ctx) & UFFD_FEATURE_INITIALIZED; } static bool userfaultfd_wp_async_ctx(struct userfaultfd_ctx *ctx) { - return ctx && (ctx->features & UFFD_FEATURE_WP_ASYNC); + return ctx && (userfaultfd_features(ctx) & UFFD_FEATURE_WP_ASYNC); } static bool userfaultfd_minor_anon_ctx(struct userfaultfd_ctx *ctx) { - return ctx && (ctx->features & UFFD_FEATURE_MINOR_ANON); + return ctx && (userfaultfd_features(ctx) & UFFD_FEATURE_MINOR_ANON); } static bool userfaultfd_minor_async_ctx(struct userfaultfd_ctx *ctx) { - return ctx && (ctx->features & UFFD_FEATURE_MINOR_ASYNC); + return ctx && (userfaultfd_features(ctx) & UFFD_FEATURE_MINOR_ASYNC); } static unsigned int userfaultfd_ctx_flags(struct userfaultfd_ctx *ctx) @@ -122,7 +131,7 @@ bool userfaultfd_wp_unpopulated(struct vm_area_struct *vma) if (!ctx) return false; - return ctx->features & UFFD_FEATURE_WP_UNPOPULATED; + return userfaultfd_features(ctx) & UFFD_FEATURE_WP_UNPOPULATED; } static int userfaultfd_wake_function(wait_queue_entry_t *wq, unsigned mode, @@ -435,7 +444,7 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason) /* 0 or > 1 flags set is a bug; we expect exactly 1. */ VM_WARN_ON_ONCE(!reason || (reason & (reason - 1))); - if (ctx->features & UFFD_FEATURE_SIGBUS) + if (userfaultfd_features(ctx) & UFFD_FEATURE_SIGBUS) goto out; if (!(vmf->flags & FAULT_FLAG_USER) && (ctx->flags & UFFD_USER_MODE_ONLY)) goto out; @@ -506,7 +515,7 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason) init_waitqueue_func_entry(&uwq.wq, userfaultfd_wake_function); uwq.wq.private = current; uwq.msg = userfault_msg(vmf->address, vmf->real_address, vmf->flags, - reason, ctx->features); + reason, userfaultfd_features(ctx)); uwq.ctx = ctx; uwq.waken = false; @@ -668,7 +677,7 @@ int dup_userfaultfd(struct vm_area_struct *vma, struct list_head *fcs) if (!octx) return 0; - if (!(octx->features & UFFD_FEATURE_EVENT_FORK)) { + if (!(userfaultfd_features(octx) & UFFD_FEATURE_EVENT_FORK)) { userfaultfd_reset_ctx(vma); return 0; } @@ -774,7 +783,7 @@ void mremap_userfaultfd_prep(struct vm_area_struct *vma, if (!ctx) return; - if (ctx->features & UFFD_FEATURE_EVENT_REMAP) { + if (userfaultfd_features(ctx) & UFFD_FEATURE_EVENT_REMAP) { vm_ctx->ctx = ctx; userfaultfd_ctx_get(ctx); down_write(&ctx->map_changing_lock); @@ -824,7 +833,7 @@ bool userfaultfd_remove(struct vm_area_struct *vma, struct userfaultfd_wait_queue ewq; ctx = vma->vm_userfaultfd_ctx.ctx; - if (!ctx || !(ctx->features & UFFD_FEATURE_EVENT_REMOVE)) + if (!ctx || !(userfaultfd_features(ctx) & UFFD_FEATURE_EVENT_REMOVE)) return true; userfaultfd_ctx_get(ctx); @@ -863,7 +872,7 @@ int userfaultfd_unmap_prep(struct vm_area_struct *vma, unsigned long start, struct userfaultfd_unmap_ctx *unmap_ctx; struct userfaultfd_ctx *ctx = vma->vm_userfaultfd_ctx.ctx; - if (!ctx || !(ctx->features & UFFD_FEATURE_EVENT_UNMAP) || + if (!ctx || !(userfaultfd_features(ctx) & UFFD_FEATURE_EVENT_UNMAP) || has_unmap_ctx(ctx, unmaps, start, end)) return 0; @@ -1826,6 +1835,65 @@ static int userfaultfd_deactivate(struct userfaultfd_ctx *ctx, return ret; } +/* + * Features that can be toggled at runtime via UFFDIO_SET_MODE. + * Only async features that were enabled at UFFDIO_API time may be toggled. + */ +#define UFFD_FEATURE_TOGGLEABLE (UFFD_FEATURE_MINOR_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 are allowed */ + if ((mode.enable | mode.disable) & ~UFFD_FEATURE_TOGGLEABLE) + return -EINVAL; + + if (!mmget_not_zero(mm)) + return -ESRCH; + + /* + * mmap_write_lock serializes against all page faults. + * After we release, no in-flight faults from the old mode exist. + */ + { + unsigned int new_features; + + mmap_write_lock(mm); + new_features = userfaultfd_features(ctx); + new_features |= mode.enable; + new_features &= ~mode.disable; + WRITE_ONCE(ctx->features, new_features); + 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_MINOR_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) { @@ -2150,6 +2218,9 @@ static long userfaultfd_ioctl(struct file *file, unsigned cmd, case UFFDIO_DEACTIVATE: ret = userfaultfd_deactivate(ctx, arg); break; + case UFFDIO_SET_MODE: + ret = userfaultfd_set_mode(ctx, arg); + break; } return ret; } @@ -2177,7 +2248,7 @@ 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, userfaultfd_features(ctx), UFFD_API_IOCTLS|UFFD_API_RANGE_IOCTLS); } #endif diff --git a/include/uapi/linux/userfaultfd.h b/include/uapi/linux/userfaultfd.h index 775825da2596..f0f14f9db06c 100644 --- a/include/uapi/linux/userfaultfd.h +++ b/include/uapi/linux/userfaultfd.h @@ -84,6 +84,7 @@ #define _UFFDIO_CONTINUE (0x07) #define _UFFDIO_POISON (0x08) #define _UFFDIO_DEACTIVATE (0x09) +#define _UFFDIO_SET_MODE (0x0A) #define _UFFDIO_API (0x3F) /* userfaultfd ioctl ids */ @@ -110,6 +111,8 @@ struct uffdio_poison) #define UFFDIO_DEACTIVATE _IOR(UFFDIO, _UFFDIO_DEACTIVATE, \ struct uffdio_range) +#define UFFDIO_SET_MODE _IOW(UFFDIO, _UFFDIO_SET_MODE, \ + struct uffdio_set_mode) /* read() structure */ struct uffd_msg { @@ -395,6 +398,16 @@ struct uffdio_move { __s64 move; }; +struct uffdio_set_mode { + /* + * Toggle async mode for features at runtime. + * Supported: UFFD_FEATURE_MINOR_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
