Re: [PATCH v4 0/4] mm/userfaultfd: modulize memory types

[Mike Rapoport <rppt@kernel.org>]

On Mon, Nov 03, 2025 at 10:27:05PM +0100, David Hildenbrand (Red Hat) wrote:
> 
> And maybe that's the main problem here: Liam talks about general uffd
> cleanups while you are focused on supporting guest_memfd minor mode "as
> simple as possible" (as you write below).

Hijacking for the technical part for a moment ;-)

It seems that "as simple as possible" can even avoid data members in struct
vm_uffd_ops, e.g something along these lines:

diff --git a/include/linux/mm.h b/include/linux/mm.h
index d16b33bacc32..840986780cb5 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -605,6 +605,8 @@ 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
@@ -690,6 +692,9 @@ 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 c0e716aec26a..aac7ac616636 100644
--- a/include/linux/userfaultfd_k.h
+++ b/include/linux/userfaultfd_k.h
@@ -111,6 +111,11 @@ static inline uffd_flags_t uffd_flags_set_mode(uffd_flags_t flags, enum mfill_at
 /* Flags controlling behavior. These behavior changes are mode-independent. */
 #define MFILL_ATOMIC_WP MFILL_ATOMIC_FLAG(0)
 
+struct vm_uffd_ops {
+	int (*minor_get_folio)(struct inode *inode, pgoff_t pgoff,
+			       struct folio **folio);
+};
+
 extern int mfill_atomic_install_pte(pmd_t *dst_pmd,
 				    struct vm_area_struct *dst_vma,
 				    unsigned long dst_addr, struct page *page,
diff --git a/mm/shmem.c b/mm/shmem.c
index b9081b817d28..b4318ad3bdf9 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -3260,6 +3260,17 @@ int shmem_mfill_atomic_pte(pmd_t *dst_pmd,
 	shmem_inode_unacct_blocks(inode, 1);
 	return ret;
 }
+
+static int shmem_uffd_minor_get_folio(struct inode *inode, pgoff_t pgoff,
+				      struct folio **folio)
+{
+	return shmem_get_folio(inode, pgoff, 0, folio, SGP_NOALLOC);
+}
+
+static const struct vm_uffd_ops shmem_uffd_ops = {
+	.minor_get_folio = shmem_uffd_minor_get_folio,
+};
+
 #endif /* CONFIG_USERFAULTFD */
 
 #ifdef CONFIG_TMPFS
@@ -5292,6 +5303,9 @@ 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 = {
@@ -5301,6 +5315,9 @@ 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 af61b95c89e4..6b30a8f39f4d 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -20,6 +20,20 @@
 #include "internal.h"
 #include "swap.h"
 
+static const struct vm_uffd_ops anon_uffd_ops = {
+};
+
+static inline const struct vm_uffd_ops *vma_get_uffd_ops(struct vm_area_struct *vma)
+{
+	if (vma->vm_ops && vma->vm_ops->uffd_ops)
+		return vma->vm_ops->uffd_ops;
+
+	if (vma_is_anonymous(vma))
+		return &anon_uffd_ops;
+
+	return NULL;
+}
+
 static __always_inline
 bool validate_dst_vma(struct vm_area_struct *dst_vma, unsigned long dst_end)
 {
@@ -382,13 +396,14 @@ static int mfill_atomic_pte_continue(pmd_t *dst_pmd,
 				     unsigned long dst_addr,
 				     uffd_flags_t flags)
 {
+	const struct vm_uffd_ops *uffd_ops = vma_get_uffd_ops(dst_vma);
 	struct inode *inode = file_inode(dst_vma->vm_file);
 	pgoff_t pgoff = linear_page_index(dst_vma, dst_addr);
 	struct folio *folio;
 	struct page *page;
 	int ret;
 
-	ret = shmem_get_folio(inode, pgoff, 0, &folio, SGP_NOALLOC);
+	ret = uffd_ops->minor_get_folio(inode, pgoff, &folio);
 	/* Our caller expects us to return -EFAULT if we failed to find folio */
 	if (ret == -ENOENT)
 		ret = -EFAULT;
@@ -707,6 +722,7 @@ static __always_inline ssize_t mfill_atomic(struct userfaultfd_ctx *ctx,
 	unsigned long src_addr, dst_addr;
 	long copied;
 	struct folio *folio;
+	const struct vm_uffd_ops *uffd_ops;
 
 	/*
 	 * Sanitize the command parameters:
@@ -766,10 +782,11 @@ static __always_inline ssize_t mfill_atomic(struct userfaultfd_ctx *ctx,
 		return  mfill_atomic_hugetlb(ctx, dst_vma, dst_start,
 					     src_start, len, flags);
 
-	if (!vma_is_anonymous(dst_vma) && !vma_is_shmem(dst_vma))
+	uffd_ops = vma_get_uffd_ops(dst_vma);
+	if (!uffd_ops)
 		goto out_unlock;
-	if (!vma_is_shmem(dst_vma) &&
-	    uffd_flags_mode_is(flags, MFILL_ATOMIC_CONTINUE))
+	if (uffd_flags_mode_is(flags, MFILL_ATOMIC_CONTINUE) &&
+	    !uffd_ops->minor_get_folio)
 		goto out_unlock;
 
 	while (src_addr < src_start + len) {
 
-- 
Sincerely yours,
Mike.