diff --git a/changes/bug20530 b/changes/bug20530
new file mode 100644
index 0000000000..c21d5fbd34
--- /dev/null
+++ b/changes/bug20530
@@ -0,0 +1,4 @@
+  o Minor Fixes (Windows):
+    - Check for getpagesize before using it to mmap files. This fixes
+      compilation in some MinGW environments. Fixes bug 20530; bugfix on
+      commit bf72878 in tor-0.1.2.1-alpha, reported by "ice".
diff --git a/configure.ac b/configure.ac
index 27d7577a5a..8ee34070a3 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1410,6 +1410,14 @@ AC_CHECK_DECLS([mlockall], , , [
 #include <sys/mman.h>
 #endif])
 
+# Some MinGW environments don't have getpagesize in unistd.h. We don't use
+# AC_CHECK_FUNCS(getpagesize), because other environments rename getpagesize
+# using macros
+AC_CHECK_DECLS([getpagesize], , , [
+#ifdef HAVE_UNISTD_H
+#include <unistd.h>
+#endif])
+
 # Allow user to specify an alternate syslog facility
 AC_ARG_WITH(syslog-facility,
 AS_HELP_STRING(--with-syslog-facility=LOG, [syslog facility to use (default=LOG_DAEMON)]),
diff --git a/src/common/compat.c b/src/common/compat.c
index 8d6a491c42..ebf05f59e1 100644
--- a/src/common/compat.c
+++ b/src/common/compat.c
@@ -204,7 +204,15 @@ tor_rename(const char *path_old, const char *path_new)
                 sandbox_intern_string(path_new));
 }
 
-#if defined(HAVE_SYS_MMAN_H) || defined(RUNNING_DOXYGEN)
+/* Some MinGW builds have sys/mman.h, but not the corresponding symbols.
+ * Other configs rename the symbols using macros (including getpagesize).
+ * So check for sys/mman.h and unistd.h, and a getpagesize declaration. */
+#if (defined(HAVE_SYS_MMAN_H) && defined(HAVE_UNISTD_H) && \
+     defined(HAVE_DECL_GETPAGESIZE))
+#define COMPAT_HAS_MMAN_AND_PAGESIZE
+#endif
+
+#if defined(COMPAT_HAS_MMAN_AND_PAGESIZE) || defined(RUNNING_DOXYGEN)
 /** Try to create a memory mapping for <b>filename</b> and return it.  On
  * failure, return NULL.  Sets errno properly, using ERANGE to mean
  * "empty file". */