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/20240327090835.3232629-1-zhangpeng362@huawei.com
Signed-off-by: ZhangPeng <zhangpeng362@huawei.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Lokesh Gidra <lokeshgidra@google.com>
Cc: Nanyong Sun <sunnanyong@huawei.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 60dcfafdc11a84d0d6ebe74ebc99c90ff145a35a..3e6ddda6f159606e33ac1c64006e40c5c851bc88 100644 (file)
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -657,7 +657,10 @@ int dup_userfaultfd(struct vm_area_struct *vma, struct list_head *fcs)
        struct userfaultfd_fork_ctx *fctx;
 
        octx = vma->vm_userfaultfd_ctx.ctx;
-       if (!octx || !(octx->features & UFFD_FEATURE_EVENT_FORK)) {
+       if (!octx)
+               return 0;
+
+       if (!(octx->features & UFFD_FEATURE_EVENT_FORK)) {
                vma_start_write(vma);
                vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
                userfaultfd_set_vm_flags(vma, vma->vm_flags & ~__VM_UFFD_FLAGS);