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@redhat.com>
Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>
---
 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 */

 #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