Implement mlock(), kind of.

Currently mlock() and friends do nothing whatsoever. However, mlocking
is directly application-visible in a number of ways; for example,
madvise(MADV_DONTNEED) and msync(MS_INVALIDATE) both fail on mlocked
regions. We handle this inconsistently: MADV_DONTNEED is too important
to not work, but MS_INVALIDATE is rejected.

Change MM to track mlocked regions in a manner consistent with Linux.
It still will not actually pin pages into host physical memory, but:

- mlock() will now cause sentry memory management to precommit mlocked
pages.

- MADV_DONTNEED and MS_INVALIDATE will interact with mlocked pages as
described above.

PiperOrigin-RevId: 225861605
Change-Id: Iee187204979ac9a4d15d0e037c152c0902c8d0ee

1 files changed, 14 insertions, 6 deletions

diff --git a/test/syscalls/linux/msync.cc b/test/syscalls/linux/msync.cc
index 0ddc621aa..72d90dc78 100644
--- a/test/syscalls/linux/msync.cc
+++ b/test/syscalls/linux/msync.cc
@@ -43,14 +43,13 @@ class MsyncParameterizedTest : public ::testing::TestWithParam<MsyncTestParam> {
  protected:
   int msync_flags() const { return std::get<0>(GetParam()); }
 
-  PosixErrorOr<Mapping> GetMapping() const {
-    auto rv = std::get<1>(GetParam())();
-    return rv;
-  }
+  PosixErrorOr<Mapping> GetMapping() const { return std::get<1>(GetParam())(); }
 };
 
-// All valid msync(2) flag combinations (not including MS_INVALIDATE, which
-// gVisor doesn't implement).
+// All valid msync(2) flag combinations, not including MS_INVALIDATE. ("Linux
+// permits a call to msync() that specifies neither [MS_SYNC or MS_ASYNC], with
+// semantics that are (currently) equivalent to specifying MS_ASYNC." -
+// msync(2))
 constexpr std::initializer_list<int> kMsyncFlags = {MS_SYNC, MS_ASYNC, 0};
 
 // Returns functions that return mappings that should be successfully
@@ -134,6 +133,15 @@ TEST_P(MsyncFullParamTest, UnalignedAddressFails) {
       SyscallFailsWithErrno(EINVAL));
 }
 
+TEST_P(MsyncFullParamTest, InvalidateUnlockedSucceeds) {
+  auto m = ASSERT_NO_ERRNO_AND_VALUE(GetMapping());
+  EXPECT_THAT(msync(m.ptr(), m.len(), msync_flags() | MS_INVALIDATE),
+              SyscallSucceeds());
+}
+
+// The test for MS_INVALIDATE on mlocked pages is in mlock.cc since it requires
+// probing for mlock support.
+
 INSTANTIATE_TEST_CASE_P(
     All, MsyncFullParamTest,
     ::testing::Combine(::testing::ValuesIn(kMsyncFlags),