[PATCH 7/9] userfaultfd: add UFFDIO_CONTINUE ioctl

[Axel Rasmussen <axelrasmussen@google.com>]

This ioctl is how userspace ought to resolve "minor" userfaults. The
idea is, userspace is notified that a minor fault has occurred. It might
change the contents of the page using its second non-UFFD mapping, or
not. Then, it calls UFFDIO_CONTINUE to tell the kernel "I have ensured
the page contents are correct, carry on setting up the mapping".

Note that it doesn't make much sense to use UFFDIO_{COPY,ZEROPAGE} for
MINOR registered VMAs. ZEROPAGE maps the VMA to the zero page; but in
the minor fault case, we already have some pre-existing underlying page.
Likewise, UFFDIO_COPY isn't useful if we have a second non-UFFD mapping.
We'd just use memcpy() or similar instead.

It turns out hugetlb_mcopy_atomic_pte() already does very close to what
we want, if an existing page is provided via `struct page **pagep`. We
already special-case the behavior a bit for the UFFDIO_ZEROPAGE case, so
just extend that design: add an enum for the three modes of operation,
and make the small adjustments needed for the MCOPY_ATOMIC_CONTINUE
case. (Basically, look up the existing page, and avoid adding the
existing page to the page cache or calling set_page_huge_active() on
it.)

Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>
---
 fs/userfaultfd.c                 | 67 +++++++++++++++++++++++++
 include/linux/userfaultfd_k.h    |  2 +
 include/uapi/linux/userfaultfd.h | 21 +++++++-
 mm/hugetlb.c                     | 11 ++--
 mm/userfaultfd.c                 | 86 ++++++++++++++++++++++++--------
 5 files changed, 158 insertions(+), 29 deletions(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 19d6925be03f..f0eb2d63289f 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -1530,6 +1530,10 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx,
 		if (!(uffdio_register.mode & UFFDIO_REGISTER_MODE_WP))
 			ioctls_out &= ~((__u64)1 << _UFFDIO_WRITEPROTECT);
 
+		/* CONTINUE ioctl is only supported for MINOR ranges. */
+		if (!(uffdio_register.mode & UFFDIO_REGISTER_MODE_MINOR))
+			ioctls_out &= ~((__u64)1 << _UFFDIO_CONTINUE);
+
 		/*
 		 * Now that we scanned all vmas we can already tell
 		 * userland which ioctls methods are guaranteed to
@@ -1883,6 +1887,66 @@ static int userfaultfd_writeprotect(struct userfaultfd_ctx *ctx,
 	return ret;
 }
 
+static int userfaultfd_continue(struct userfaultfd_ctx *ctx, unsigned long arg)
+{
+	__s64 ret;
+	struct uffdio_continue uffdio_continue;
+	struct uffdio_continue __user *user_uffdio_continue;
+	struct userfaultfd_wake_range range;
+
+	user_uffdio_continue = (struct uffdio_continue __user *)arg;
+
+	ret = -EAGAIN;
+	if (READ_ONCE(ctx->mmap_changing))
+		goto out;
+
+	ret = -EFAULT;
+	if (copy_from_user(&uffdio_continue, user_uffdio_continue,
+			   /* don't copy the output fields */
+			   sizeof(uffdio_continue) - (sizeof(__s64))))
+		goto out;
+
+	ret = validate_range(ctx->mm, &uffdio_continue.range.start,
+			     uffdio_continue.range.len);
+	if (ret)
+		goto out;
+
+	ret = -EINVAL;
+	/* double check for wraparound just in case. */
+	if (uffdio_continue.range.start + uffdio_continue.range.len <=
+	    uffdio_continue.range.start) {
+		goto out;
+	}
+	if (uffdio_continue.mode & ~UFFDIO_CONTINUE_MODE_DONTWAKE)
+		goto out;
+
+	if (mmget_not_zero(ctx->mm)) {
+		ret = mcopy_continue(ctx->mm, uffdio_continue.range.start,
+				     uffdio_continue.range.len,
+				     &ctx->mmap_changing);
+		mmput(ctx->mm);
+	} else {
+		return -ESRCH;
+	}
+
+	if (unlikely(put_user(ret, &user_uffdio_continue->mapped)))
+		return -EFAULT;
+	if (ret < 0)
+		goto out;
+
+	/* len == 0 would wake all */
+	BUG_ON(!ret);
+	range.len = ret;
+	if (!(uffdio_continue.mode & UFFDIO_CONTINUE_MODE_DONTWAKE)) {
+		range.start = uffdio_continue.range.start;
+		wake_userfault(ctx, &range);
+	}
+	ret = range.len == uffdio_continue.range.len ? 0 : -EAGAIN;
+
+out:
+	return ret;
+}
+
 static inline unsigned int uffd_ctx_features(__u64 user_features)
 {
 	/*
@@ -1967,6 +2031,9 @@ static long userfaultfd_ioctl(struct file *file, unsigned cmd,
 	case UFFDIO_WRITEPROTECT:
 		ret = userfaultfd_writeprotect(ctx, arg);
 		break;
+	case UFFDIO_CONTINUE:
+		ret = userfaultfd_continue(ctx, arg);
+		break;
 	}
 	return ret;
 }
diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
index ed157804ca02..49d7e7b4581f 100644
--- a/include/linux/userfaultfd_k.h
+++ b/include/linux/userfaultfd_k.h
@@ -41,6 +41,8 @@ extern ssize_t mfill_zeropage(struct mm_struct *dst_mm,
 			      unsigned long dst_start,
 			      unsigned long len,
 			      bool *mmap_changing);
+extern ssize_t mcopy_continue(struct mm_struct *dst_mm, unsigned long dst_start,
+			      unsigned long len, bool *mmap_changing);
 extern int mwriteprotect_range(struct mm_struct *dst_mm,
 			       unsigned long start, unsigned long len,
 			       bool enable_wp, bool *mmap_changing);
diff --git a/include/uapi/linux/userfaultfd.h b/include/uapi/linux/userfaultfd.h
index 1cc2cd8a5279..9a48305f4bdd 100644
--- a/include/uapi/linux/userfaultfd.h
+++ b/include/uapi/linux/userfaultfd.h
@@ -40,10 +40,12 @@
 	((__u64)1 << _UFFDIO_WAKE |		\
 	 (__u64)1 << _UFFDIO_COPY |		\
 	 (__u64)1 << _UFFDIO_ZEROPAGE |		\
-	 (__u64)1 << _UFFDIO_WRITEPROTECT)
+	 (__u64)1 << _UFFDIO_WRITEPROTECT |	\
+	 (__u64)1 << _UFFDIO_CONTINUE)
 #define UFFD_API_RANGE_IOCTLS_BASIC		\
 	((__u64)1 << _UFFDIO_WAKE |		\
-	 (__u64)1 << _UFFDIO_COPY)
+	 (__u64)1 << _UFFDIO_COPY |		\
+	 (__u64)1 << _UFFDIO_CONTINUE)
 
 /*
  * Valid ioctl command number range with this API is from 0x00 to
@@ -59,6 +61,7 @@
 #define _UFFDIO_COPY			(0x03)
 #define _UFFDIO_ZEROPAGE		(0x04)
 #define _UFFDIO_WRITEPROTECT		(0x06)
+#define _UFFDIO_CONTINUE		(0x07)
 #define _UFFDIO_API			(0x3F)
 
 /* userfaultfd ioctl ids */
@@ -77,6 +80,8 @@
 				      struct uffdio_zeropage)
 #define UFFDIO_WRITEPROTECT	_IOWR(UFFDIO, _UFFDIO_WRITEPROTECT, \
 				      struct uffdio_writeprotect)
+#define UFFDIO_CONTINUE		_IOR(UFFDIO, _UFFDIO_CONTINUE,	\
+				     struct uffdio_continue)
 
 /* read() structure */
 struct uffd_msg {
@@ -268,6 +273,18 @@ struct uffdio_writeprotect {
 	__u64 mode;
 };
 
+struct uffdio_continue {
+	struct uffdio_range range;
+#define UFFDIO_CONTINUE_MODE_DONTWAKE		((__u64)1<<0)
+	__u64 mode;
+
+	/*
+	 * Fields below here are written by the ioctl and must be at the end:
+	 * the copy_from_user will not read past here.
+	 */
+	__s64 mapped;
+};
+
 /*
  * Flags for the userfaultfd(2) system call itself.
  */
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 2b3741d6130c..84392d5fa079 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -4666,12 +4666,14 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
 	spinlock_t *ptl;
 	int ret;
 	struct page *page;
+	bool new_page = false;
 
 	if (!*pagep) {
 		ret = -ENOMEM;
 		page = alloc_huge_page(dst_vma, dst_addr, 0);
 		if (IS_ERR(page))
 			goto out;
+		new_page = true;
 
 		ret = copy_huge_page_from_user(page,
 						(const void __user *) src_addr,
@@ -4699,10 +4701,8 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
 	mapping = dst_vma->vm_file->f_mapping;
 	idx = vma_hugecache_offset(h, dst_vma, dst_addr);
 
-	/*
-	 * If shared, add to page cache
-	 */
-	if (vm_shared) {
+	/* Add shared, newly allocated pages to the page cache. */
+	if (vm_shared && new_page) {
 		size = i_size_read(mapping->host) >> huge_page_shift(h);
 		ret = -EFAULT;
 		if (idx >= size)
@@ -4762,7 +4762,8 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
 	update_mmu_cache(dst_vma, dst_addr, dst_pte);
 
 	spin_unlock(ptl);
-	set_page_huge_active(page);
+	if (new_page)
+		set_page_huge_active(page);
 	if (vm_shared)
 		unlock_page(page);
 	ret = 0;
diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index b2ce61c1b50d..0ecc50525dd4 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -197,6 +197,16 @@ static pmd_t *mm_alloc_pmd(struct mm_struct *mm, unsigned long address)
 	return pmd_alloc(mm, pud, address);
 }
 
+/* The mode of operation for __mcopy_atomic and its helpers. */
+enum mcopy_atomic_mode {
+	/* A normal copy_from_user into the destination range. */
+	MCOPY_ATOMIC_NORMAL,
+	/* Don't copy; map the destination range to the zero page. */
+	MCOPY_ATOMIC_ZEROPAGE,
+	/* Just setup the dst_vma, without modifying the underlying page(s). */
+	MCOPY_ATOMIC_CONTINUE,
+};
+
 #ifdef CONFIG_HUGETLB_PAGE
 /*
  * __mcopy_atomic processing for HUGETLB vmas.  Note that this routine is
@@ -207,7 +217,7 @@ static __always_inline ssize_t __mcopy_atomic_hugetlb(struct mm_struct *dst_mm,
 					      unsigned long dst_start,
 					      unsigned long src_start,
 					      unsigned long len,
-					      bool zeropage)
+					      enum mcopy_atomic_mode mode)
 {
 	int vm_alloc_shared = dst_vma->vm_flags & VM_SHARED;
 	int vm_shared = dst_vma->vm_flags & VM_SHARED;
@@ -227,7 +237,7 @@ static __always_inline ssize_t __mcopy_atomic_hugetlb(struct mm_struct *dst_mm,
 	 * by THP.  Since we can not reliably insert a zero page, this
 	 * feature is not supported.
 	 */
-	if (zeropage) {
+	if (mode == MCOPY_ATOMIC_ZEROPAGE) {
 		mmap_read_unlock(dst_mm);
 		return -EINVAL;
 	}
@@ -273,8 +283,6 @@ static __always_inline ssize_t __mcopy_atomic_hugetlb(struct mm_struct *dst_mm,
 	}
 
 	while (src_addr < src_start + len) {
-		pte_t dst_pteval;
-
 		BUG_ON(dst_addr >= dst_start + len);
 
 		/*
@@ -297,12 +305,22 @@ static __always_inline ssize_t __mcopy_atomic_hugetlb(struct mm_struct *dst_mm,
 			goto out_unlock;
 		}
 
-		err = -EEXIST;
-		dst_pteval = huge_ptep_get(dst_pte);
-		if (!huge_pte_none(dst_pteval)) {
-			mutex_unlock(&hugetlb_fault_mutex_table[hash]);
-			i_mmap_unlock_read(mapping);
-			goto out_unlock;
+		if (mode == MCOPY_ATOMIC_CONTINUE) {
+			/* hugetlb_mcopy_atomic_pte unlocks the page. */
+			page = find_lock_page(mapping, idx);
+			if (!page) {
+				err = -EFAULT;
+				mutex_unlock(&hugetlb_fault_mutex_table[hash]);
+				i_mmap_unlock_read(mapping);
+				goto out_unlock;
+			}
+		} else {
+			if (!huge_pte_none(huge_ptep_get(dst_pte))) {
+				err = -EEXIST;
+				mutex_unlock(&hugetlb_fault_mutex_table[hash]);
+				i_mmap_unlock_read(mapping);
+				goto out_unlock;
+			}
 		}
 
 		err = hugetlb_mcopy_atomic_pte(dst_mm, dst_pte, dst_vma,
@@ -408,7 +426,7 @@ extern ssize_t __mcopy_atomic_hugetlb(struct mm_struct *dst_mm,
 				      unsigned long dst_start,
 				      unsigned long src_start,
 				      unsigned long len,
-				      bool zeropage);
+				      enum mcopy_atomic_mode mode);
 #endif /* CONFIG_HUGETLB_PAGE */
 
 static __always_inline ssize_t mfill_atomic_pte(struct mm_struct *dst_mm,
@@ -417,7 +435,7 @@ static __always_inline ssize_t mfill_atomic_pte(struct mm_struct *dst_mm,
 						unsigned long dst_addr,
 						unsigned long src_addr,
 						struct page **page,
-						bool zeropage,
+						enum mcopy_atomic_mode mode,
 						bool wp_copy)
 {
 	ssize_t err;
@@ -433,22 +451,38 @@ static __always_inline ssize_t mfill_atomic_pte(struct mm_struct *dst_mm,
 	 * and not in the radix tree.
 	 */
 	if (!(dst_vma->vm_flags & VM_SHARED)) {
-		if (!zeropage)
+		switch (mode) {
+		case MCOPY_ATOMIC_NORMAL:
 			err = mcopy_atomic_pte(dst_mm, dst_pmd, dst_vma,
 					       dst_addr, src_addr, page,
 					       wp_copy);
-		else
+			break;
+		case MCOPY_ATOMIC_ZEROPAGE:
 			err = mfill_zeropage_pte(dst_mm, dst_pmd,
 						 dst_vma, dst_addr);
+			break;
+		/* It only makes sense to CONTINUE for shared memory. */
+		case MCOPY_ATOMIC_CONTINUE:
+			err = -EINVAL;
+			break;
+		}
 	} else {
 		VM_WARN_ON_ONCE(wp_copy);
-		if (!zeropage)
+		switch (mode) {
+		case MCOPY_ATOMIC_NORMAL:
 			err = shmem_mcopy_atomic_pte(dst_mm, dst_pmd,
 						     dst_vma, dst_addr,
 						     src_addr, page);
-		else
+			break;
+		case MCOPY_ATOMIC_ZEROPAGE:
 			err = shmem_mfill_zeropage_pte(dst_mm, dst_pmd,
 						       dst_vma, dst_addr);
+			break;
+		case MCOPY_ATOMIC_CONTINUE:
+			/* FIXME: Add minor fault interception for shmem. */
+			err = -EINVAL;
+			break;
+		}
 	}
 
 	return err;
@@ -458,7 +492,7 @@ static __always_inline ssize_t __mcopy_atomic(struct mm_struct *dst_mm,
 					      unsigned long dst_start,
 					      unsigned long src_start,
 					      unsigned long len,
-					      bool zeropage,
+					      enum mcopy_atomic_mode mcopy_mode,
 					      bool *mmap_changing,
 					      __u64 mode)
 {
@@ -527,7 +561,7 @@ static __always_inline ssize_t __mcopy_atomic(struct mm_struct *dst_mm,
 	 */
 	if (is_vm_hugetlb_page(dst_vma))
 		return  __mcopy_atomic_hugetlb(dst_mm, dst_vma, dst_start,
-						src_start, len, zeropage);
+						src_start, len, mcopy_mode);
 
 	if (!vma_is_anonymous(dst_vma) && !vma_is_shmem(dst_vma))
 		goto out_unlock;
@@ -577,7 +611,7 @@ static __always_inline ssize_t __mcopy_atomic(struct mm_struct *dst_mm,
 		BUG_ON(pmd_trans_huge(*dst_pmd));
 
 		err = mfill_atomic_pte(dst_mm, dst_pmd, dst_vma, dst_addr,
-				       src_addr, &page, zeropage, wp_copy);
+				       src_addr, &page, mcopy_mode, wp_copy);
 		cond_resched();
 
 		if (unlikely(err == -ENOENT)) {
@@ -626,14 +660,22 @@ ssize_t mcopy_atomic(struct mm_struct *dst_mm, unsigned long dst_start,
 		     unsigned long src_start, unsigned long len,
 		     bool *mmap_changing, __u64 mode)
 {
-	return __mcopy_atomic(dst_mm, dst_start, src_start, len, false,
-			      mmap_changing, mode);
+	return __mcopy_atomic(dst_mm, dst_start, src_start, len,
+			      MCOPY_ATOMIC_NORMAL, mmap_changing, mode);
 }
 
 ssize_t mfill_zeropage(struct mm_struct *dst_mm, unsigned long start,
 		       unsigned long len, bool *mmap_changing)
 {
-	return __mcopy_atomic(dst_mm, start, 0, len, true, mmap_changing, 0);
+	return __mcopy_atomic(dst_mm, start, 0, len, MCOPY_ATOMIC_ZEROPAGE,
+			      mmap_changing, 0);
+}
+
+ssize_t mcopy_continue(struct mm_struct *dst_mm, unsigned long start,
+		       unsigned long len, bool *mmap_changing)
+{
+	return __mcopy_atomic(dst_mm, start, 0, len, MCOPY_ATOMIC_CONTINUE,
+			      mmap_changing, 0);
 }
 
 int mwriteprotect_range(struct mm_struct *dst_mm, unsigned long start,
-- 
2.30.0.284.gd98b1dd5eaa7-goog