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authorNadav Amit <namit@vmware.com>2021-09-02 14:59:02 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2021-09-03 09:58:16 -0700
commit4410cbb5c9f9371556c4e928b5dd00226b073082 (patch)
treee7daee670f9d38dc6ea5c3406a3bd9a26635f82b /tools
parent22e5fe2a2a279d9a6fcbdfb4dffe73821bef1c90 (diff)
selftests/vm/userfaultfd: wake after copy failure
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. Link: https://lkml.kernel.org/r/20210808020724.1022515-4-namit@vmware.com Signed-off-by: Nadav Amit <namit@vmware.com> 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: Axel Rasmussen <axelrasmussen@google.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>
Diffstat (limited to 'tools')
-rw-r--r--tools/testing/selftests/vm/userfaultfd.c13
1 files changed, 13 insertions, 0 deletions
diff --git a/tools/testing/selftests/vm/userfaultfd.c b/tools/testing/selftests/vm/userfaultfd.c
index 2ea438e6b8b1..10ab56c2484a 100644
--- a/tools/testing/selftests/vm/userfaultfd.c
+++ b/tools/testing/selftests/vm/userfaultfd.c
@@ -566,6 +566,18 @@ 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;
@@ -585,6 +597,7 @@ static int __copy_page(int ufd, unsigned long offset, bool retry)
if (uffdio_copy.copy != -EEXIST)
err("UFFDIO_COPY error: %"PRId64,
(int64_t)uffdio_copy.copy);
+ wake_range(ufd, uffdio_copy.dst, page_size);
} else if (uffdio_copy.copy != page_size) {
err("UFFDIO_COPY error: %"PRId64, (int64_t)uffdio_copy.copy);
} else {