userfaultfd: early return in dup_userfaultfd()

When vma->vm_userfaultfd_ctx.ctx is NULL, vma->vm_flags should have
cleared __VM_UFFD_FLAGS. Therefore, there is no need to down_write or
clear the flag, which will affect fork performance. Fix this by
returning early if octx is NULL in dup_userfaultfd().

By applying this patch we can get a 1.3% performance improvement for
lmbench fork_prot. Results are as follows:
                   base      early return
Process fork+exit: 419.1106  413.4804

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: ZhangPeng <[email protected]>
Reviewed-by: Peter Xu <[email protected]>
Cc: Axel Rasmussen <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Kefeng Wang <[email protected]>
Cc: Liam R. Howlett <[email protected]>
Cc: Lokesh Gidra <[email protected]>
Cc: Nanyong Sun <[email protected]>
Cc: Suren Baghdasaryan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

0 files changed, 0 insertions, 0 deletions