Allow failed MADV_REMOVEs when the only target mappings (if any) are …

…MAP_PRIVATE

tcmalloc does this and it's perfectly fine because such MADV_REMOVEs have no effect.

CMakeLists.txt

@@ -1076,9 +1076,10 @@ set(BASIC_TESTS
   legacy_ugid
   x86/lsl
   madvise
-  madvise_free
   madvise_dontneed_private
+  madvise_free
   madvise_misc
+  madvise_remove
   madvise_wipeonfork
   map_fixed
   map_shared_syscall

src/record_syscall.cc

@@ -6488,10 +6488,21 @@ static void record_madvise(RecordTask* t) {
         ranges.insert(ranges.end(), pages_present.begin(), pages_present.end());
       }
       break;
-    case MADV_REMOVE:
-      // We don't handle this yet...
-      ASSERT(t, false) << "Possibly-partial MADV_REMOVEs not handled yet";
+    case MADV_REMOVE: {
+      for (const auto& m : t->vm()->maps_containing_or_after(start)) {
+        if (m.map.start() >= end) {
+          break;
+        }
+        if (m.map.flags() & MAP_PRIVATE) {
+          // Private mappings fail cleanly with MADV_REMOVE so we
+          // don't need to worry about any effects on them.
+          continue;
+        }
+        // We don't handle this yet...
+        ASSERT(t, false) << "Possibly-partial MADV_REMOVEs not handled yet";
+      }
       break;
+    }
     default:
       break;
   }

src/test/madvise_remove.c

@@ -0,0 +1,18 @@
+/* -*- Mode: C; tab-width: 8; c-basic-offset: 2; indent-tabs-mode: nil; -*- */
+
+#include "util.h"
+
+int main(void) {
+  size_t page_size = sysconf(_SC_PAGESIZE);
+  char* p =
+      mmap(NULL, page_size*3, PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
+  test_assert(p != MAP_FAILED);
+  test_assert(0 == munmap(p + page_size, page_size));
+
+  test_assert(-1 == madvise(p, page_size, MADV_REMOVE));
+  test_assert(errno == EINVAL);
+
+  atomic_puts("EXIT-SUCCESS");
+
+  return 0;
+}