[PATCH RFC 13/17] mm: introduce VM_FAULT_UFFD_MISSING fault reason

[Mike Rapoport <rppt@kernel.org>]

From: Nikita Kalyazin <kalyazin@amazon.com>

When a VMA is registered with userfaulfd in missing mode, its ->fault()
method should check if a folio exists in the page cache and if no
->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_MISSING 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_MISSING there instead.

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

Signed-off-by: Nikita Kalyazin <kalyazin@amazon.com>
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
---
 include/linux/mm_types.h | 7 ++++++-
 mm/memory.c              | 5 ++++-
 mm/shmem.c               | 2 +-
 3 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index b25ac322bfbf..a061c43e835b 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -1557,6 +1557,8 @@ typedef __bitwise unsigned int vm_fault_t;
  * @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_UFFD_MISSING:	->fault did not modify page tables and needs
+ *				handle_userfault(VM_UFFD_MISSING) to complete
  * @VM_FAULT_HINDEX_MASK:	mask HINDEX value
  *
  */
@@ -1576,8 +1578,10 @@ enum vm_fault_reason {
 	VM_FAULT_COMPLETED      = (__force vm_fault_t)0x004000,
 #ifdef CONFIG_USERFAULTFD
 	VM_FAULT_UFFD_MINOR	= (__force vm_fault_t)0x008000,
+	VM_FAULT_UFFD_MISSING	= (__force vm_fault_t)0x010000,
 #else
 	VM_FAULT_UFFD_MINOR	= (__force vm_fault_t)0x000000,
+	VM_FAULT_UFFD_MISSING	= (__force vm_fault_t)0x000000,
 #endif
 	VM_FAULT_HINDEX_MASK    = (__force vm_fault_t)0x0f0000,
 };
@@ -1604,7 +1608,8 @@ enum vm_fault_reason {
 	{ VM_FAULT_DONE_COW,            "DONE_COW" },	\
 	{ VM_FAULT_NEEDDSYNC,           "NEEDDSYNC" },	\
 	{ VM_FAULT_COMPLETED,           "COMPLETED" },	\
-	{ VM_FAULT_UFFD_MINOR,		"UFFD_MINOR" }
+	{ VM_FAULT_UFFD_MINOR,		"UFFD_MINOR" }, \
+	{ VM_FAULT_UFFD_MISSING,	"UFFD_MISSING" }
 
 struct vm_special_mapping {
 	const char *name;	/* The name, e.g. "[vdso]". */
diff --git a/mm/memory.c b/mm/memory.c
index fcb3e0c3113e..f72e69a43b68 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -5319,9 +5319,12 @@ static vm_fault_t __do_fault(struct vm_fault *vmf)
 
 	ret = vma->vm_ops->fault(vmf);
 	if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE | VM_FAULT_RETRY |
-			    VM_FAULT_DONE_COW | VM_FAULT_UFFD_MINOR))) {
+			    VM_FAULT_DONE_COW | VM_FAULT_UFFD_MINOR |
+			    VM_FAULT_UFFD_MISSING))) {
 		if (ret & VM_FAULT_UFFD_MINOR)
 			return handle_userfault(vmf, VM_UFFD_MINOR);
+		if (ret & VM_FAULT_UFFD_MISSING)
+			return handle_userfault(vmf, VM_UFFD_MISSING);
 		return ret;
 	}
 
diff --git a/mm/shmem.c b/mm/shmem.c
index 6aa905147c0c..1bc544cab2a8 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2530,7 +2530,7 @@ static int shmem_get_folio_gfp(struct inode *inode, pgoff_t index,
 	 */
 
 	if (vma && userfaultfd_missing(vma)) {
-		*fault_type = handle_userfault(vmf, VM_UFFD_MISSING);
+		*fault_type = VM_FAULT_UFFD_MISSING;
 		return 0;
 	}
 
-- 
2.51.0