On 11/23/25 11:27, Mike Rapoport wrote:
From: "Mike Rapoport (Microsoft)" <[email protected]>
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) <[email protected]>
Signed-off-by: Mike Rapoport (Microsoft) <[email protected]>
---
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.
--
Cheers
David
