[PATCH RFC 07/17] userfaultfd: introduce vm_uffd_ops

[Mike Rapoport <rppt@kernel.org>]

From: "Mike Rapoport (Microsoft)" <rppt@kernel.org>

Current userfaultfd implementation works only with memory managed by
core MM: anonymous, shmem and hugetlb.

First, there is no fundamental reason to limit userfaultfd support only
to the core memory types and userfaults can be handled similarly to
regular page faults provided a VMA owner implements appropriate
callbacks.

Second, historically various code paths were conditioned on
vma_is_anonymous(), vma_is_shmem() and is_vm_hugetlb_page() and some of
these conditions can be expressed as operations implemented by a
particular memory type.

Introduce vm_uffd_ops extension to vm_operations_struct that will
delegate memory type specific operations to a VMA owner.

Operations for anonymous memory are handled internally in userfaultfd
using anon_uffd_ops that implicitly assigned to anonymous VMAs.

Start with a single operation, ->can_userfault() that will verify that a
VMA meets requirements for userfaultfd support at registration time.

Implement that method for anonymous, shmem and hugetlb and move relevant
parts of vma_can_userfault() into the new callbacks.

Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
---
 include/linux/mm.h            |  5 +++++
 include/linux/userfaultfd_k.h |  6 +++++
 mm/hugetlb.c                  | 21 ++++++++++++++++++
 mm/shmem.c                    | 23 ++++++++++++++++++++
 mm/userfaultfd.c              | 41 ++++++++++++++++++++++-------------
 5 files changed, 81 insertions(+), 15 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 15076261d0c2..3c2caff646c3 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -732,6 +732,8 @@ struct vm_fault {
 					 */
 };
 
+struct vm_uffd_ops;
+
 /*
  * These are the virtual MM functions - opening of an area, closing and
  * unmapping it (needed to keep files on disk up-to-date etc), pointer
@@ -817,6 +819,9 @@ struct vm_operations_struct {
 	struct page *(*find_normal_page)(struct vm_area_struct *vma,
 					 unsigned long addr);
 #endif /* CONFIG_FIND_NORMAL_PAGE */
+#ifdef CONFIG_USERFAULTFD
+	const struct vm_uffd_ops *uffd_ops;
+#endif
 };
 
 #ifdef CONFIG_NUMA_BALANCING
diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
index a49cf750e803..56e85ab166c7 100644
--- a/include/linux/userfaultfd_k.h
+++ b/include/linux/userfaultfd_k.h
@@ -80,6 +80,12 @@ struct userfaultfd_ctx {
 
 extern vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason);
 
+/* VMA userfaultfd operations */
+struct vm_uffd_ops {
+	/* Checks if a VMA can support userfaultfd */
+	bool (*can_userfault)(struct vm_area_struct *vma, vm_flags_t vm_flags);
+};
+
 /* A combined operation mode + behavior flags. */
 typedef unsigned int __bitwise uffd_flags_t;
 
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 51273baec9e5..909131910c43 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -4797,6 +4797,24 @@ static vm_fault_t hugetlb_vm_op_fault(struct vm_fault *vmf)
 	return 0;
 }
 
+#ifdef CONFIG_USERFAULTFD
+static bool hugetlb_can_userfault(struct vm_area_struct *vma,
+				  vm_flags_t vm_flags)
+{
+	/*
+	 * If user requested uffd-wp but not enabled pte markers for
+	 * uffd-wp, then hugetlb is not supported.
+	 */
+	if (!uffd_supports_wp_marker() && (vm_flags & VM_UFFD_WP))
+		return false;
+	return true;
+}
+
+static const struct vm_uffd_ops hugetlb_uffd_ops = {
+	.can_userfault = hugetlb_can_userfault,
+};
+#endif
+
 /*
  * When a new function is introduced to vm_operations_struct and added
  * to hugetlb_vm_ops, please consider adding the function to shm_vm_ops.
@@ -4810,6 +4828,9 @@ const struct vm_operations_struct hugetlb_vm_ops = {
 	.close = hugetlb_vm_op_close,
 	.may_split = hugetlb_vm_op_split,
 	.pagesize = hugetlb_vm_op_pagesize,
+#ifdef CONFIG_USERFAULTFD
+	.uffd_ops = &hugetlb_uffd_ops,
+#endif
 };
 
 static pte_t make_huge_pte(struct vm_area_struct *vma, struct folio *folio,
diff --git a/mm/shmem.c b/mm/shmem.c
index ec6c01378e9d..9b82cda271c4 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -5290,6 +5290,23 @@ static const struct super_operations shmem_ops = {
 #endif
 };
 
+#ifdef CONFIG_USERFAULTFD
+static bool shmem_can_userfault(struct vm_area_struct *vma, vm_flags_t vm_flags)
+{
+	/*
+	 * If user requested uffd-wp but not enabled pte markers for
+	 * uffd-wp, then shmem is not supported.
+	 */
+	if (!uffd_supports_wp_marker() && (vm_flags & VM_UFFD_WP))
+		return false;
+	return true;
+}
+
+static const struct vm_uffd_ops shmem_uffd_ops = {
+	.can_userfault	= shmem_can_userfault,
+};
+#endif
+
 static const struct vm_operations_struct shmem_vm_ops = {
 	.fault		= shmem_fault,
 	.map_pages	= filemap_map_pages,
@@ -5297,6 +5314,9 @@ static const struct vm_operations_struct shmem_vm_ops = {
 	.set_policy     = shmem_set_policy,
 	.get_policy     = shmem_get_policy,
 #endif
+#ifdef CONFIG_USERFAULTFD
+	.uffd_ops	= &shmem_uffd_ops,
+#endif
 };
 
 static const struct vm_operations_struct shmem_anon_vm_ops = {
@@ -5306,6 +5326,9 @@ static const struct vm_operations_struct shmem_anon_vm_ops = {
 	.set_policy     = shmem_set_policy,
 	.get_policy     = shmem_get_policy,
 #endif
+#ifdef CONFIG_USERFAULTFD
+	.uffd_ops	= &shmem_uffd_ops,
+#endif
 };
 
 int shmem_init_fs_context(struct fs_context *fc)
diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 786f0a245675..d035f5e17f07 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -34,6 +34,25 @@ struct mfill_state {
 	pmd_t *pmd;
 };
 
+static bool anon_can_userfault(struct vm_area_struct *vma, vm_flags_t vm_flags)
+{
+	/* anonymous memory does not support MINOR mode */
+	if (vm_flags & VM_UFFD_MINOR)
+		return false;
+	return true;
+}
+
+static const struct vm_uffd_ops anon_uffd_ops = {
+	.can_userfault	= anon_can_userfault,
+};
+
+static const struct vm_uffd_ops *vma_uffd_ops(struct vm_area_struct *vma)
+{
+	if (vma_is_anonymous(vma))
+		return &anon_uffd_ops;
+	return vma->vm_ops ? vma->vm_ops->uffd_ops : NULL;
+}
+
 static __always_inline
 bool validate_dst_vma(struct vm_area_struct *dst_vma, unsigned long dst_end)
 {
@@ -2019,13 +2038,15 @@ ssize_t move_pages(struct userfaultfd_ctx *ctx, unsigned long dst_start,
 bool vma_can_userfault(struct vm_area_struct *vma, vm_flags_t vm_flags,
 		       bool wp_async)
 {
-	vm_flags &= __VM_UFFD_FLAGS;
+	const struct vm_uffd_ops *ops = vma_uffd_ops(vma);
 
-	if (vma->vm_flags & VM_DROPPABLE)
+	/* only VMAs that implement vm_uffd_ops are supported */
+	if (!ops)
 		return false;
 
-	if ((vm_flags & VM_UFFD_MINOR) &&
-	    (!is_vm_hugetlb_page(vma) && !vma_is_shmem(vma)))
+	vm_flags &= __VM_UFFD_FLAGS;
+
+	if (vma->vm_flags & VM_DROPPABLE)
 		return false;
 
 	/*
@@ -2035,18 +2056,8 @@ bool vma_can_userfault(struct vm_area_struct *vma, vm_flags_t vm_flags,
 	if (wp_async && (vm_flags == VM_UFFD_WP))
 		return true;
 
-	/*
-	 * If user requested uffd-wp but not enabled pte markers for
-	 * uffd-wp, then shmem & hugetlbfs are not supported but only
-	 * anonymous.
-	 */
-	if (!uffd_supports_wp_marker() && (vm_flags & VM_UFFD_WP) &&
-	    !vma_is_anonymous(vma))
-		return false;
-
 	/* By default, allow any of anon|shmem|hugetlb */
-	return vma_is_anonymous(vma) || is_vm_hugetlb_page(vma) ||
-	    vma_is_shmem(vma);
+	return ops->can_userfault(vma, vm_flags);
 }
 
 static void userfaultfd_set_vm_flags(struct vm_area_struct *vma,
-- 
2.51.0