[PATCH v2 3/5] mm: introduce VM_FAULT_UFFD_MINOR fault reason

[Mike Rapoport <rppt@kernel.org>]

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

When a VMA is registered with userfaulfd in minor mode, its ->fault()
method should check if a folio exists in the page cache and if yes
->fault() should call handle_userfault(VM_UFFD_MISSING).

Instead of calling handle_userfault() directly from a specific ->fault()
implementation introduce new fault reason VM_FAULT_UFFD_MINOR that will
notify the core page fault handler that it should call
handle_userfaultfd(VM_UFFD_MISSING) to complete a page fault.

Replace a call to handle_userfault(VM_UFFD_MISSING) in shmem and use the
new VM_FAULT_UFFD_MINOR there instead.

For configurations that don't enable CONFIG_USERFAULTFD,
VM_FAULT_UFFD_MINOR is set to 0.

Suggested-by: David Hildenbrand (Red Hat) <david@kernel.org>
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
---
 include/linux/mm_types.h | 10 +++++++++-
 mm/memory.c              |  2 ++
 mm/shmem.c               |  2 +-
 3 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 90e5790c318f..df71b057111b 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -1523,6 +1523,8 @@ typedef __bitwise unsigned int vm_fault_t;
  *				fsync() to complete (for synchronous page faults
  *				in DAX)
  * @VM_FAULT_COMPLETED:		->fault completed, meanwhile mmap lock released
+ * @VM_FAULT_UFFD_MINOR:	->fault did not modify page tables and needs
+ *				handle_userfault(VM_UFFD_MINOR) to complete
  * @VM_FAULT_HINDEX_MASK:	mask HINDEX value
  *
  */
@@ -1540,6 +1542,11 @@ enum vm_fault_reason {
 	VM_FAULT_DONE_COW       = (__force vm_fault_t)0x001000,
 	VM_FAULT_NEEDDSYNC      = (__force vm_fault_t)0x002000,
 	VM_FAULT_COMPLETED      = (__force vm_fault_t)0x004000,
+#ifdef CONFIG_USERFAULTFD
+	VM_FAULT_UFFD_MINOR	= (__force vm_fault_t)0x008000,
+#else
+	VM_FAULT_UFFD_MINOR	= (__force vm_fault_t)0x000000,
+#endif
 	VM_FAULT_HINDEX_MASK    = (__force vm_fault_t)0x0f0000,
 };
 
@@ -1564,7 +1571,8 @@ enum vm_fault_reason {
 	{ VM_FAULT_FALLBACK,            "FALLBACK" },	\
 	{ VM_FAULT_DONE_COW,            "DONE_COW" },	\
 	{ VM_FAULT_NEEDDSYNC,           "NEEDDSYNC" },	\
-	{ VM_FAULT_COMPLETED,           "COMPLETED" }
+	{ VM_FAULT_COMPLETED,           "COMPLETED" },	\
+	{ VM_FAULT_UFFD_MINOR,		"UFFD_MINOR" },	\
 
 struct vm_special_mapping {
 	const char *name;	/* The name, e.g. "[vdso]". */
diff --git a/mm/memory.c b/mm/memory.c
index b59ae7ce42eb..94acbac8cefb 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -5279,6 +5279,8 @@ static vm_fault_t __do_fault(struct vm_fault *vmf)
 	}
 
 	ret = vma->vm_ops->fault(vmf);
+	if (unlikely(ret & VM_FAULT_UFFD_MINOR))
+		return handle_userfault(vmf, VM_UFFD_MINOR);
 	if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE | VM_FAULT_RETRY |
 			    VM_FAULT_DONE_COW)))
 		return ret;
diff --git a/mm/shmem.c b/mm/shmem.c
index e16c7c8c3e1e..a9a31c0b5979 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2461,7 +2461,7 @@ static int shmem_get_folio_gfp(struct inode *inode, pgoff_t index,
 	if (folio && vma && userfaultfd_minor(vma)) {
 		if (!xa_is_value(folio))
 			folio_put(folio);
-		*fault_type = handle_userfault(vmf, VM_UFFD_MINOR);
+		*fault_type = VM_FAULT_UFFD_MINOR;
 		return 0;
 	}
 
-- 
2.50.1