Currently, most of the userfaultfd features are implemented directly in the
core mm.  It will invoke VMA specific functions whenever necessary.  So far
it is fine because it almost only interacts with shmem and hugetlbfs.

Introduce a generic userfaultfd API extension for vm_operations_struct,
so that any code that implements vm_operations_struct (including kernel
modules that can be compiled separately from the kernel core) can support
userfaults without modifying the core files.

With this API applied, if a module wants to support userfaultfd, the
module should only need to properly define vm_uffd_ops and hook it to
vm_operations_struct, instead of changing anything in core mm.

This API will not work for anonymous memory. Handling of userfault
operations for anonymous memory remains unchanged in core mm.

Due to a security concern while reviewing older versions of this series
[1], uffd_copy() will be temprorarily removed.  IOW, so far MISSING-capable
memory types can only be hard-coded and implemented in mm/.  It would also
affect UFFDIO_COPY and UFFDIO_ZEROPAGE.  Other functions should still be
able to be provided from vm_uffd_ops.

Introduces the API only so that existing userfaultfd users can be moved
over without breaking them.

[1] https://lore.kernel.org/all/20250627154655.2085903-1-peterx@redhat.com/

Signed-off-by: Peter Xu <peterx@redhat.com>
---
 include/linux/mm.h            |  9 +++++++++
 include/linux/userfaultfd_k.h | 37 +++++++++++++++++++++++++++++++++++
 2 files changed, 46 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 6b6c6980f46c2..8afb93387e2c6 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -620,6 +620,8 @@ 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
@@ -705,6 +707,13 @@ 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
+	/*
+	 * Userfaultfd related ops.  Modules need to define this to support
+	 * userfaultfd.
+	 */
+	const struct vm_uffd_ops *userfaultfd_ops;
+#endif
 };
 
 #ifdef CONFIG_NUMA_BALANCING
diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
index c0e716aec26aa..b1949d8611238 100644
--- a/include/linux/userfaultfd_k.h
+++ b/include/linux/userfaultfd_k.h
@@ -92,6 +92,43 @@ enum mfill_atomic_mode {
 	NR_MFILL_ATOMIC_MODES,
 };
 
+/* VMA userfaultfd operations */
+struct vm_uffd_ops {
+	/**
+	 * @uffd_features: features supported in bitmask.
+	 *
+	 * When the ops is defined, the driver must set non-zero features
+	 * to be a subset (or all) of: VM_UFFD_MISSING|WP|MINOR.
+	 *
+	 * NOTE: VM_UFFD_MISSING is still only supported under mm/ so far.
+	 */
+	unsigned long uffd_features;
+	/**
+	 * @uffd_ioctls: ioctls supported in bitmask.
+	 *
+	 * Userfaultfd ioctls supported by the module.  Below will always
+	 * be supported by default whenever a module provides vm_uffd_ops:
+	 *
+	 *   _UFFDIO_API, _UFFDIO_REGISTER, _UFFDIO_UNREGISTER, _UFFDIO_WAKE
+	 *
+	 * The module needs to provide all the rest optionally supported
+	 * ioctls.  For example, when VM_UFFD_MINOR is supported,
+	 * _UFFDIO_CONTINUE must be supported as an ioctl.
+	 */
+	unsigned long uffd_ioctls;
+	/**
+	 * uffd_get_folio: Handler to resolve UFFDIO_CONTINUE request.
+	 *
+	 * @inode: the inode for folio lookup
+	 * @pgoff: the pgoff of the folio
+	 * @folio: returned folio pointer
+	 *
+	 * Return: zero if succeeded, negative for errors.
+	 */
+	int (*uffd_get_folio)(struct inode *inode, pgoff_t pgoff,
+			      struct folio **folio);
+};
+
 #define MFILL_ATOMIC_MODE_BITS (const_ilog2(NR_MFILL_ATOMIC_MODES - 1) + 1)
 #define MFILL_ATOMIC_BIT(nr) BIT(MFILL_ATOMIC_MODE_BITS + (nr))
 #define MFILL_ATOMIC_FLAG(nr) ((__force uffd_flags_t) MFILL_ATOMIC_BIT(nr))
-- 
2.50.1