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

[David Hildenbrand (Red Hat)]

@@ -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)))

If we want to reduce the overhead on the fast path, we can simply do

if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE | VM_FAULT_RETRY |
                    VM_FAULT_DONE_COW | VM_FAULT_UFFD_MINOR))) {
        if (unlikely(ret & VM_FAULT_UFFD_MINOR))
                return handle_userfault(vmf, VM_UFFD_MINOR);
        return ret;
}

Maybe the compiler already does that to improve the likely case.

LGTM

--
Cheers

David