[RFC PATCH 03/13] selftests/vm/userfaultfd: wake after copy failure

[Nadav Amit <nadav.amit@gmail.com>]

From: Nadav Amit <namit@vmware.com>

When userfaultfd copy-ioctl fails since the PTE already exists, an
-EEXIST error is returned and the faulting thread is not woken. The
current userfaultfd test does not wake the faulting thread in such case.
The assumption is presumably that another thread set the PTE through
copy/wp ioctl and would wake the faulting thread or that alternatively
the fault handler would realize there is no need to "must_wait" and
continue. This is not necessarily true.

There is an assumption that the "must_wait" tests in handle_userfault()
are sufficient to provide definitive answer whether the offending PTE is
populated or not. However, userfaultfd_must_wait() test is lockless.
Consequently, concurrent calls to ptep_modify_prot_start(), for
instance, can clear the PTE and can cause userfaultfd_must_wait()
to wrongly assume it is not populated and a wait is needed.

There are therefore 3 options:
(1) Change the tests to wake on copy failure.
(2) Wake faulting thread unconditionally on zero/copy ioctls before
    returning -EEXIST.
(3) Change the userfaultfd_must_wait() to hold locks.

This patch took the first approach, but the others are valid solutions
with different tradeoffs.

Cc: Jens Axboe <axboe@kernel.dk>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: io-uring@vger.kernel.org
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: linux-mm@kvack.org
Signed-off-by: Nadav Amit <namit@vmware.com>
---
 tools/testing/selftests/vm/userfaultfd.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/tools/testing/selftests/vm/userfaultfd.c b/tools/testing/selftests/vm/userfaultfd.c
index 9b0912a01777..f7e6cf43db71 100644
--- a/tools/testing/selftests/vm/userfaultfd.c
+++ b/tools/testing/selftests/vm/userfaultfd.c
@@ -484,6 +484,18 @@ static void retry_copy_page(int ufd, struct uffdio_copy *uffdio_copy,
 	}
 }
 
+static void wake_range(int ufd, unsigned long addr, unsigned long len)
+{
+	struct uffdio_range uffdio_wake;
+
+	uffdio_wake.start = addr;
+	uffdio_wake.len = len;
+
+	if (ioctl(ufd, UFFDIO_WAKE, &uffdio_wake))
+		fprintf(stderr, "error waking %lu\n",
+			addr), exit(1);
+}
+
 static int __copy_page(int ufd, unsigned long offset, bool retry)
 {
 	struct uffdio_copy uffdio_copy;
@@ -507,6 +519,7 @@ static int __copy_page(int ufd, unsigned long offset, bool retry)
 				uffdio_copy.copy);
 			exit(1);
 		}
+		wake_range(ufd, uffdio_copy.dst, page_size);
 	} else if (uffdio_copy.copy != page_size) {
 		fprintf(stderr, "UFFDIO_COPY unexpected copy %Ld\n",
 			uffdio_copy.copy); exit(1);
-- 
2.25.1