On Wed, Feb 10, 2021 at 1:22 PM Axel Rasmussen <axelrasmussen@xxxxxxxxxx> wrote:
>
> This feature allows userspace to intercept "minor" faults. By "minor"
> faults, I mean the following situation:
>
> Let there exist two mappings (i.e., VMAs) to the same page(s). 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.
>
> This commit adds the new registration mode, and sets the relevant flag
> on the VMAs being registered. In the hugetlb fault path, if we find
> that we have huge_pte_none(), but find_lock_page() does indeed find an
> existing page, then we have a "minor" fault, and if the VMA has the
> userfaultfd registration flag, we call into userfaultfd to handle it.
>
> Why add a new registration mode, as opposed to adding a feature to
> MISSING registration, like UFFD_FEATURE_SIGBUS?
>
> - The semantics are significantly different. UFFDIO_COPY or
> UFFDIO_ZEROPAGE do not make sense for these minor faults; userspace
> would instead just memset() or memcpy() or whatever via the non-UFFD
> mapping. Unlike MISSING registration, MINOR registration only makes
> sense for hugetlbfs (or, in the future, shmem), as this is the only
> way to get two VMAs to a single set of underlying pages.
>
> - Doing so would make handle_userfault()'s "reason" argument confusing.
> We'd pass in "MISSING" even if the pages weren't really missing.
>
> Reviewed-by: Peter Xu <peterx@xxxxxxxxxx>
> Signed-off-by: Axel Rasmussen <axelrasmussen@xxxxxxxxxx>
> ---
> fs/proc/task_mmu.c | 1 +
> fs/userfaultfd.c | 71 ++++++++++++++++++--------------
> include/linux/mm.h | 1 +
> include/linux/userfaultfd_k.h | 15 ++++++-
> include/trace/events/mmflags.h | 1 +
> include/uapi/linux/userfaultfd.h | 15 ++++++-
> mm/hugetlb.c | 32 ++++++++++++++
> 7 files changed, 102 insertions(+), 34 deletions(-)
>
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 602e3a52884d..94e951ea3e03 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -651,6 +651,7 @@ static void show_smap_vma_flags(struct seq_file *m, struct vm_area_struct *vma)
> [ilog2(VM_MTE)] = "mt",
> [ilog2(VM_MTE_ALLOWED)] = "",
> #endif
> + [ilog2(VM_UFFD_MINOR)] = "ui",
> #ifdef CONFIG_ARCH_HAS_PKEYS
> /* These come out via ProtectionKey: */
> [ilog2(VM_PKEY_BIT0)] = "",
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index 1f4a34b1a1e7..b351a8552140 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -197,24 +197,21 @@ static inline struct uffd_msg userfault_msg(unsigned long address,
> msg_init(&msg);
> msg.event = UFFD_EVENT_PAGEFAULT;
> msg.arg.pagefault.address = address;
> + /*
> + * These flags indicate why the userfault occurred:
> + * - UFFD_PAGEFAULT_FLAG_WP indicates a write protect fault.
> + * - UFFD_PAGEFAULT_FLAG_MINOR indicates a minor fault.
> + * - Neither of these flags being set indicates a MISSING fault.
> + *
> + * Separately, UFFD_PAGEFAULT_FLAG_WRITE indicates it was a write
> + * fault. Otherwise, it was a read fault.
> + */
> if (flags & FAULT_FLAG_WRITE)
> - /*
> - * If UFFD_FEATURE_PAGEFAULT_FLAG_WP was set in the
> - * uffdio_api.features and UFFD_PAGEFAULT_FLAG_WRITE
> - * was not set in a UFFD_EVENT_PAGEFAULT, it means it
> - * was a read fault, otherwise if set it means it's
> - * a write fault.
> - */
> msg.arg.pagefault.flags |= UFFD_PAGEFAULT_FLAG_WRITE;
> if (reason & VM_UFFD_WP)
> - /*
> - * If UFFD_FEATURE_PAGEFAULT_FLAG_WP was set in the
> - * uffdio_api.features and UFFD_PAGEFAULT_FLAG_WP was
> - * not set in a UFFD_EVENT_PAGEFAULT, it means it was
> - * a missing fault, otherwise if set it means it's a
> - * write protect fault.
> - */
> msg.arg.pagefault.flags |= UFFD_PAGEFAULT_FLAG_WP;
> + if (reason & VM_UFFD_MINOR)
> + msg.arg.pagefault.flags |= UFFD_PAGEFAULT_FLAG_MINOR;
> if (features & UFFD_FEATURE_THREAD_ID)
> msg.arg.pagefault.feat.ptid = task_pid_vnr(current);
> return msg;
> @@ -401,8 +398,10 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
>
> BUG_ON(ctx->mm != mm);
>
> - VM_BUG_ON(reason & ~(VM_UFFD_MISSING|VM_UFFD_WP));
> - VM_BUG_ON(!(reason & VM_UFFD_MISSING) ^ !!(reason & VM_UFFD_WP));
> + /* Any unrecognized flag is a bug. */
> + VM_BUG_ON(reason & ~__VM_UFFD_FLAGS);
> + /* 0 or > 1 flags set is a bug; we expect exactly 1. */
> + VM_BUG_ON(!reason || !!(reason & (reason - 1)));
>
> if (ctx->features & UFFD_FEATURE_SIGBUS)
> goto out;
> @@ -612,7 +611,7 @@ static void userfaultfd_event_wait_completion(struct userfaultfd_ctx *ctx,
> for (vma = mm->mmap; vma; vma = vma->vm_next)
> if (vma->vm_userfaultfd_ctx.ctx == release_new_ctx) {
> vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
> - vma->vm_flags &= ~(VM_UFFD_WP | VM_UFFD_MISSING);
> + vma->vm_flags &= ~__VM_UFFD_FLAGS;
> }
> mmap_write_unlock(mm);
>
> @@ -644,7 +643,7 @@ int dup_userfaultfd(struct vm_area_struct *vma, struct list_head *fcs)
> octx = vma->vm_userfaultfd_ctx.ctx;
> if (!octx || !(octx->features & UFFD_FEATURE_EVENT_FORK)) {
> vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
> - vma->vm_flags &= ~(VM_UFFD_WP | VM_UFFD_MISSING);
> + vma->vm_flags &= ~__VM_UFFD_FLAGS;
> return 0;
> }
>
> @@ -726,7 +725,7 @@ void mremap_userfaultfd_prep(struct vm_area_struct *vma,
> } else {
> /* Drop uffd context if remap feature not enabled */
> vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
> - vma->vm_flags &= ~(VM_UFFD_WP | VM_UFFD_MISSING);
> + vma->vm_flags &= ~__VM_UFFD_FLAGS;
> }
> }
>
> @@ -867,12 +866,12 @@ static int userfaultfd_release(struct inode *inode, struct file *file)
> for (vma = mm->mmap; vma; vma = vma->vm_next) {
> cond_resched();
> BUG_ON(!!vma->vm_userfaultfd_ctx.ctx ^
> - !!(vma->vm_flags & (VM_UFFD_MISSING | VM_UFFD_WP)));
> + !!(vma->vm_flags & __VM_UFFD_FLAGS));
> if (vma->vm_userfaultfd_ctx.ctx != ctx) {
> prev = vma;
> continue;
> }
> - new_flags = vma->vm_flags & ~(VM_UFFD_MISSING | VM_UFFD_WP);
> + new_flags = vma->vm_flags & ~__VM_UFFD_FLAGS;
> prev = vma_merge(mm, prev, vma->vm_start, vma->vm_end,
> new_flags, vma->anon_vma,
> vma->vm_file, vma->vm_pgoff,
> @@ -1306,9 +1305,19 @@ static inline bool vma_can_userfault(struct vm_area_struct *vma,
> unsigned long vm_flags)
> {
> /* FIXME: add WP support to hugetlbfs and shmem */
> - return vma_is_anonymous(vma) ||
> - ((is_vm_hugetlb_page(vma) || vma_is_shmem(vma)) &&
> - !(vm_flags & VM_UFFD_WP));
> + if (vm_flags & VM_UFFD_WP) {
> + if (is_vm_hugetlb_page(vma) || vma_is_shmem(vma))
> + return false;
> + }
> +
> + if (vm_flags & VM_UFFD_MINOR) {
> + /* FIXME: Add minor fault interception for shmem. */
> + if (!is_vm_hugetlb_page(vma))
> + return false;
> + }
> +
> + return vma_is_anonymous(vma) || is_vm_hugetlb_page(vma) ||
> + vma_is_shmem(vma);
> }
>
> static int userfaultfd_register(struct userfaultfd_ctx *ctx,
> @@ -1334,14 +1343,15 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx,
> ret = -EINVAL;
> if (!uffdio_register.mode)
> goto out;
> - if (uffdio_register.mode & ~(UFFDIO_REGISTER_MODE_MISSING|
> - UFFDIO_REGISTER_MODE_WP))
> + if (uffdio_register.mode & ~UFFD_API_REGISTER_MODES)
> goto out;
> vm_flags = 0;
> if (uffdio_register.mode & UFFDIO_REGISTER_MODE_MISSING)
> vm_flags |= VM_UFFD_MISSING;
> if (uffdio_register.mode & UFFDIO_REGISTER_MODE_WP)
> vm_flags |= VM_UFFD_WP;
> + if (uffdio_register.mode & UFFDIO_REGISTER_MODE_MINOR)
> + vm_flags |= VM_UFFD_MINOR;
>
> ret = validate_range(mm, &uffdio_register.range.start,
> uffdio_register.range.len);
> @@ -1385,7 +1395,7 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx,
> cond_resched();
>
> BUG_ON(!!cur->vm_userfaultfd_ctx.ctx ^
> - !!(cur->vm_flags & (VM_UFFD_MISSING | VM_UFFD_WP)));
> + !!(cur->vm_flags & __VM_UFFD_FLAGS));
>
> /* check not compatible vmas */
> ret = -EINVAL;
> @@ -1465,8 +1475,7 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx,
> start = vma->vm_start;
> vma_end = min(end, vma->vm_end);
>
> - new_flags = (vma->vm_flags &
> - ~(VM_UFFD_MISSING|VM_UFFD_WP)) | vm_flags;
> + new_flags = (vma->vm_flags & ~__VM_UFFD_FLAGS) | vm_flags;
> prev = vma_merge(mm, prev, start, vma_end, new_flags,
> vma->anon_vma, vma->vm_file, vma->vm_pgoff,
> vma_policy(vma),
> @@ -1588,7 +1597,7 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
> cond_resched();
>
> BUG_ON(!!cur->vm_userfaultfd_ctx.ctx ^
> - !!(cur->vm_flags & (VM_UFFD_MISSING | VM_UFFD_WP)));
> + !!(cur->vm_flags & __VM_UFFD_FLAGS));
>
> /*
> * Check not compatible vmas, not strictly required
> @@ -1639,7 +1648,7 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
> wake_userfault(vma->vm_userfaultfd_ctx.ctx, &range);
> }
>
> - new_flags = vma->vm_flags & ~(VM_UFFD_MISSING | VM_UFFD_WP);
> + new_flags = vma->vm_flags & ~__VM_UFFD_FLAGS;
> prev = vma_merge(mm, prev, start, vma_end, new_flags,
> vma->anon_vma, vma->vm_file, vma->vm_pgoff,
> vma_policy(vma),
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 89fca443e6f1..3f65a506c743 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -276,6 +276,7 @@ extern unsigned int kobjsize(const void *objp);
> #define VM_PFNMAP 0x00000400 /* Page-ranges managed without "struct page", just pure PFN */
> #define VM_DENYWRITE 0x00000800 /* ETXTBSY on write attempts.. */
> #define VM_UFFD_WP 0x00001000 /* wrprotect pages tracking */
> +#define VM_UFFD_MINOR 0x00002000 /* minor fault interception */
Ah, I had added this just after VM_UFFD_WP, without noticing that this
would be sharing a bit with VM_LOCKED. That seems like not such a
great idea.
I don't see another unused bit, and I don't see some other obvious
candidate to share with. So, the solution that comes to mind is
something like:
- Since it isn't feasible to have one VM_ flag per UFFD trigger type,
handle_userfault()'s "reason" argument should be some enumeration of
possible UFFD reasons instead.
- Introduce a path where handle_userfault() can return 0, meaning "you
called into me, but I am not meant to be handling this fault per the
userfaultfd_ctx, handle it normally instead".
- Use VM_UFFD_MISSING to decide whether or not to call
handle_userfault(), whether it was a missing or minor fault.
Unless there are objections or some simpler idea, I'll send a v6 with
this change.
>
> #define VM_LOCKED 0x00002000
> #define VM_IO 0x00004000 /* Memory mapped I/O or similar */
> diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
> index c63ccdae3eab..0390e5ac63b3 100644
> --- a/include/linux/userfaultfd_k.h
> +++ b/include/linux/userfaultfd_k.h
> @@ -17,6 +17,9 @@
> #include <linux/mm.h>
> #include <asm-generic/pgtable_uffd.h>
>
> +/* The set of all possible UFFD-related VM flags. */
> +#define __VM_UFFD_FLAGS (VM_UFFD_MISSING | VM_UFFD_WP | VM_UFFD_MINOR)
> +
> /*
> * CAREFUL: Check include/uapi/asm-generic/fcntl.h when defining
> * new flags, since they might collide with O_* ones. We want
> @@ -71,6 +74,11 @@ static inline bool userfaultfd_wp(struct vm_area_struct *vma)
> return vma->vm_flags & VM_UFFD_WP;
> }
>
> +static inline bool userfaultfd_minor(struct vm_area_struct *vma)
> +{
> + return vma->vm_flags & VM_UFFD_MINOR;
> +}
> +
> static inline bool userfaultfd_pte_wp(struct vm_area_struct *vma,
> pte_t pte)
> {
> @@ -85,7 +93,7 @@ static inline bool userfaultfd_huge_pmd_wp(struct vm_area_struct *vma,
>
> static inline bool userfaultfd_armed(struct vm_area_struct *vma)
> {
> - return vma->vm_flags & (VM_UFFD_MISSING | VM_UFFD_WP);
> + return vma->vm_flags & __VM_UFFD_FLAGS;
> }
>
> extern int dup_userfaultfd(struct vm_area_struct *, struct list_head *);
> @@ -132,6 +140,11 @@ static inline bool userfaultfd_wp(struct vm_area_struct *vma)
> return false;
> }
>
> +static inline bool userfaultfd_minor(struct vm_area_struct *vma)
> +{
> + return false;
> +}
> +
> static inline bool userfaultfd_pte_wp(struct vm_area_struct *vma,
> pte_t pte)
> {
> diff --git a/include/trace/events/mmflags.h b/include/trace/events/mmflags.h
> index 67018d367b9f..2d583ffd4100 100644
> --- a/include/trace/events/mmflags.h
> +++ b/include/trace/events/mmflags.h
> @@ -151,6 +151,7 @@ IF_HAVE_PG_ARCH_2(PG_arch_2, "arch_2" )
> {VM_PFNMAP, "pfnmap" }, \
> {VM_DENYWRITE, "denywrite" }, \
> {VM_UFFD_WP, "uffd_wp" }, \
> + {VM_UFFD_MINOR, "uffd_minor" }, \
> {VM_LOCKED, "locked" }, \
> {VM_IO, "io" }, \
> {VM_SEQ_READ, "seqread" }, \
> diff --git a/include/uapi/linux/userfaultfd.h b/include/uapi/linux/userfaultfd.h
> index 5f2d88212f7c..f24dd4fcbad9 100644
> --- a/include/uapi/linux/userfaultfd.h
> +++ b/include/uapi/linux/userfaultfd.h
> @@ -19,15 +19,19 @@
> * means the userland is reading).
> */
> #define UFFD_API ((__u64)0xAA)
> +#define UFFD_API_REGISTER_MODES (UFFDIO_REGISTER_MODE_MISSING | \
> + UFFDIO_REGISTER_MODE_WP | \
> + UFFDIO_REGISTER_MODE_MINOR)
> #define UFFD_API_FEATURES (UFFD_FEATURE_PAGEFAULT_FLAG_WP | \
> UFFD_FEATURE_EVENT_FORK | \
> UFFD_FEATURE_EVENT_REMAP | \
> - UFFD_FEATURE_EVENT_REMOVE | \
> + UFFD_FEATURE_EVENT_REMOVE | \
> UFFD_FEATURE_EVENT_UNMAP | \
> UFFD_FEATURE_MISSING_HUGETLBFS | \
> UFFD_FEATURE_MISSING_SHMEM | \
> UFFD_FEATURE_SIGBUS | \
> - UFFD_FEATURE_THREAD_ID)
> + UFFD_FEATURE_THREAD_ID | \
> + UFFD_FEATURE_MINOR_HUGETLBFS)
> #define UFFD_API_IOCTLS \
> ((__u64)1 << _UFFDIO_REGISTER | \
> (__u64)1 << _UFFDIO_UNREGISTER | \
> @@ -127,6 +131,7 @@ struct uffd_msg {
> /* flags for UFFD_EVENT_PAGEFAULT */
> #define UFFD_PAGEFAULT_FLAG_WRITE (1<<0) /* If this was a write fault */
> #define UFFD_PAGEFAULT_FLAG_WP (1<<1) /* If reason is VM_UFFD_WP */
> +#define UFFD_PAGEFAULT_FLAG_MINOR (1<<2) /* If reason is VM_UFFD_MINOR */
>
> struct uffdio_api {
> /* userland asks for an API number and the features to enable */
> @@ -171,6 +176,10 @@ struct uffdio_api {
> *
> * UFFD_FEATURE_THREAD_ID pid of the page faulted task_struct will
> * be returned, if feature is not requested 0 will be returned.
> + *
> + * UFFD_FEATURE_MINOR_HUGETLBFS indicates that minor faults
> + * can be intercepted (via REGISTER_MODE_MINOR) for
> + * hugetlbfs-backed pages.
> */
> #define UFFD_FEATURE_PAGEFAULT_FLAG_WP (1<<0)
> #define UFFD_FEATURE_EVENT_FORK (1<<1)
> @@ -181,6 +190,7 @@ struct uffdio_api {
> #define UFFD_FEATURE_EVENT_UNMAP (1<<6)
> #define UFFD_FEATURE_SIGBUS (1<<7)
> #define UFFD_FEATURE_THREAD_ID (1<<8)
> +#define UFFD_FEATURE_MINOR_HUGETLBFS (1<<9)
> __u64 features;
>
> __u64 ioctls;
> @@ -195,6 +205,7 @@ struct uffdio_register {
> struct uffdio_range range;
> #define UFFDIO_REGISTER_MODE_MISSING ((__u64)1<<0)
> #define UFFDIO_REGISTER_MODE_WP ((__u64)1<<1)
> +#define UFFDIO_REGISTER_MODE_MINOR ((__u64)1<<2)
> __u64 mode;
>
> /*
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index e41b77cf6cc2..f150b10981a8 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -4366,6 +4366,38 @@ static vm_fault_t hugetlb_no_page(struct mm_struct *mm,
> VM_FAULT_SET_HINDEX(hstate_index(h));
> goto backout_unlocked;
> }
> +
> + /* Check for page in userfault range. */
> + if (userfaultfd_minor(vma)) {
> + u32 hash;
> + struct vm_fault vmf = {
> + .vma = vma,
> + .address = haddr,
> + .flags = flags,
> + /*
> + * Hard to debug if it ends up being used by a
> + * callee that assumes something about the
> + * other uninitialized fields... same as in
> + * memory.c
> + */
> + };
> +
> + unlock_page(page);
> +
> + /*
> + * hugetlb_fault_mutex and i_mmap_rwsem must be dropped
> + * before handling userfault. Reacquire after handling
> + * fault to make calling code simpler.
> + */
> +
> + hash = hugetlb_fault_mutex_hash(mapping, idx);
> + mutex_unlock(&hugetlb_fault_mutex_table[hash]);
> + i_mmap_unlock_read(mapping);
> + ret = handle_userfault(&vmf, VM_UFFD_MINOR);
> + i_mmap_lock_read(mapping);
> + mutex_lock(&hugetlb_fault_mutex_table[hash]);
> + goto out;
> + }
> }
>
> /*
> --
> 2.30.0.478.g8a0d178c01-goog
>
