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

On 11/23/25 11:27, Mike Rapoport wrote:

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.

Suggested-by: David Hildenbrand (Red Hat) david@kernel.org Signed-off-by: Mike Rapoport (Microsoft) rppt@kernel.org

---

include/linux/mm_types.h | 3 +++ mm/memory.c | 2 ++ mm/shmem.c | 2 +- 3 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h index 90e5790c318f..eb135369940f 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 +1542,7 @@ 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,

• VM_FAULT_UFFD_MINOR = (__force vm_fault_t)0x008000, VM_FAULT_HINDEX_MASK = (__force vm_fault_t)0x0f0000, };

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 we could define VM_FAULT_UFFD_MINOR to be 0 without USERFAULTFD, we could optimize that check out completely on such configs.