On Tue, 14 Apr 2026 15:23:44 +0100 "Kiryl Shutsemau (Meta)" <[email protected]> wrote:
> 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; The commit message states "Only async features that were enabled at UFFDIO_API time may be toggled." However, the code only checks that the requested feature is in UFFD_FEATURE_TOGGLEABLE. Is it intentional that a user who opened a uffd without UFFD_FEATURE_MINOR_ASYNC can still enable it later via UFFDIO_SET_MODE? > + > + 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 > >
