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

David Hildenbrand (Red Hat)
2025-11-26T10:19:37Z

> @@ -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