Fix msync() usage in Unix filesystem implementation.

Apparently, msync()'s parameters must be page-aligned, bah.

c++/src/kj/filesystem-disk-test.c++

@@ -310,10 +310,10 @@ KJ_TEST("DiskFile") {
     KJ_EXPECT(kj::str(writableMapping->get().slice(0, 6).asChars()) == "foobaz");
     KJ_EXPECT(kj::str(privateMapping.slice(0, 6).asChars()) == "Foobaz");
 
-    writableMapping->get()[0] = 'D';
-    writableMapping->changed(writableMapping->get().slice(0, 1));
-    KJ_EXPECT(kj::str(mapping.slice(0, 6).asChars()) == "Doobaz");
-    KJ_EXPECT(kj::str(writableMapping->get().slice(0, 6).asChars()) == "Doobaz");
+    writableMapping->get()[1] = 'D';
+    writableMapping->changed(writableMapping->get().slice(1, 2));
+    KJ_EXPECT(kj::str(mapping.slice(0, 6).asChars()) == "fDobaz");
+    KJ_EXPECT(kj::str(writableMapping->get().slice(0, 6).asChars()) == "fDobaz");
     KJ_EXPECT(kj::str(privateMapping.slice(0, 6).asChars()) == "Foobaz");
 
     file->write(0, StringPtr("qux").asBytes());

c++/src/kj/filesystem-disk-unix.c++

@@ -441,13 +441,19 @@ public:
     void changed(ArrayPtr<byte> slice) override {
       KJ_REQUIRE(slice.begin() >= bytes.begin() && slice.end() <= bytes.end(),
                  "byte range is not part of this mapping");
-      KJ_SYSCALL(msync(slice.begin(), slice.size(), MS_ASYNC));
+
+      // msync() requires page-alignment, apparently, so use getMmapRange() to accomplish that.
+      auto range = getMmapRange(reinterpret_cast<uintptr_t>(slice.begin()), slice.size());
+      KJ_SYSCALL(msync(reinterpret_cast<void*>(range.offset), range.size, MS_ASYNC));
     }
 
     void sync(ArrayPtr<byte> slice) override {
       KJ_REQUIRE(slice.begin() >= bytes.begin() && slice.end() <= bytes.end(),
                  "byte range is not part of this mapping");
-      KJ_SYSCALL(msync(slice.begin(), slice.size(), MS_SYNC));
+
+      // msync() requires page-alignment, apparently, so use getMmapRange() to accomplish that.
+      auto range = getMmapRange(reinterpret_cast<uintptr_t>(slice.begin()), slice.size());
+      KJ_SYSCALL(msync(reinterpret_cast<void*>(range.offset), range.size, MS_SYNC));
     }
 
   private: