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

Mike Rapoport
2025-11-25T18:39:07Z

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