userfaultfd: call userfaultfd_unmap_prep only if __split_vma succeeds

A __split_vma is not a worthy event to report, and it's definitely not a
unmap so it would be incorrect to report unmap for the whole region to
the userfaultfd manager if a __split_vma fails.

So only call userfaultfd_unmap_prep after the __vma_splitting is over
and do_munmap cannot fail anymore.

Also add unlikely because it's better to optimize for the vast majority
of apps that aren't using userfaultfd in a non cooperative way.  Ideally
we should also find a way to eliminate the branch entirely if
CONFIG_USERFAULTFD=n, but it would complicate things so stick to
unlikely for now.

Link: http://lkml.kernel.org/r/20170802165145.22628-5-aarcange@redhat.com
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Cc: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Cc: Alexey Perevalov <a.perevalov@samsung.com>
Cc: Maxime Coquelin <maxime.coquelin@redhat.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/mmap.c

@@ -2639,13 +2639,6 @@ int do_munmap(struct mm_struct *mm, unsigned long start, size_t len,
 	if (vma->vm_start >= end)
 		return 0;
 
-	if (uf) {
-		int error = userfaultfd_unmap_prep(vma, start, end, uf);
-
-		if (error)
-			return error;
-	}
-
 	/*
 	 * If we need to split any vma, do it now to save pain later.
 	 *
@@ -2679,6 +2672,21 @@ int do_munmap(struct mm_struct *mm, unsigned long start, size_t len,
 	}
 	vma = prev ? prev->vm_next : mm->mmap;
 
+	if (unlikely(uf)) {
+		/*
+		 * If userfaultfd_unmap_prep returns an error the vmas
+		 * will remain splitted, but userland will get a
+		 * highly unexpected error anyway. This is no
+		 * different than the case where the first of the two
+		 * __split_vma fails, but we don't undo the first
+		 * split, despite we could. This is unlikely enough
+		 * failure that it's not worth optimizing it for.
+		 */
+		int error = userfaultfd_unmap_prep(vma, start, end, uf);
+		if (error)
+			return error;
+	}
+
 	/*
 	 * unlock any mlock()ed ranges before detaching vmas
 	 */