Fix: don't use overlapping mmap mappings on Cygwin

The allocation scheme used by the mmap based RCU hash table is to make a
large unaccessible mapping to reserve memory without allocating it.
Then smaller chunks are allocated by overlapping read/write mappings which
do allocate memory. Deallocation is done by an overlapping unaccessible
mapping.

This scheme was tested on Linux, macOS and Solaris. However, on Cygwin the
mmap wrapper is based on the Windows NtMapViewOfSection API which doesn't
support overlapping mappings.

An alternative to the overlapping mappings is to use mprotect to change the
protection on chunks of the large mapping, read/write to allocate and none
to deallocate. This works perfecty on Cygwin and Solaris but on Linux a
call to madvise is also required to deallocate and it just doesn't work on
macOS.

For this reason, we keep to original scheme on all platforms except Cygwin.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

diff --git a/src/rculfhash-mm-mmap.c b/src/rculfhash-mm-mmap.c
index 3cc3fa01c9d5ed5c16888ae8f02fb85ae65cf1de..a8fadf0696de5e743fe56c4190cc0e4d8e1578b1 100644 (file)
--- a/src/rculfhash-mm-mmap.c
+++ b/src/rculfhash-mm-mmap.c
@@ -28,8 +28,30 @@
 #define MAP_ANONYMOUS          MAP_ANON
 #endif
 
-/* reserve inaccessible memory space without allocation any memory */
-static void *memory_map(size_t length)
+/*
+ * The allocation scheme used by the mmap based RCU hash table is to make a
+ * large unaccessible mapping to reserve memory without allocating it.
+ * Then smaller chunks are allocated by overlapping read/write mappings which
+ * do allocate memory. Deallocation is done by an overlapping unaccessible
+ * mapping.
+ *
+ * This scheme was tested on Linux, macOS and Solaris. However, on Cygwin the
+ * mmap wrapper is based on the Windows NtMapViewOfSection API which doesn't
+ * support overlapping mappings.
+ *
+ * An alternative to the overlapping mappings is to use mprotect to change the
+ * protection on chunks of the large mapping, read/write to allocate and none
+ * to deallocate. This works perfecty on Cygwin and Solaris but on Linux a
+ * call to madvise is also required to deallocate and it just doesn't work on
+ * macOS.
+ *
+ * For this reason, we keep to original scheme on all platforms except Cygwin.
+ */
+
+
+/* Reserve inaccessible memory space without allocating it */
+static
+void *memory_map(size_t length)
 {
        void *ret = mmap(NULL, length, PROT_NONE,
                        MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
@@ -38,7 +60,8 @@ static void *memory_map(size_t length)
        return ret;
 }
 
-static void memory_unmap(void *ptr, size_t length)
+static
+void memory_unmap(void *ptr, size_t length)
 {
        int ret __attribute__((unused));
 
@@ -47,7 +70,33 @@ static void memory_unmap(void *ptr, size_t length)
        assert(ret == 0);
 }
 
-static void memory_populate(void *ptr, size_t length)
+#ifdef __CYGWIN__
+/* Set protection to read/write to allocate a memory chunk */
+static
+void memory_populate(void *ptr, size_t length)
+{
+       int ret __attribute__((unused));
+
+       ret = mprotect(ptr, length, PROT_READ | PROT_WRITE);
+
+       assert(!ret);
+}
+
+/* Set protection to none to deallocate a memory chunk */
+static
+void memory_discard(void *ptr, size_t length)
+{
+       int ret __attribute__((unused));
+
+       ret = mprotect(ptr, length, PROT_NONE);
+
+       assert(!ret);
+}
+
+#else /* __CYGWIN__ */
+
+static
+void memory_populate(void *ptr, size_t length)
 {
        void *ret __attribute__((unused));
 
@@ -61,7 +110,8 @@ static void memory_populate(void *ptr, size_t length)
  * Discard garbage memory and avoid system save it when try to swap it out.
  * Make it still reserved, inaccessible.
  */
-static void memory_discard(void *ptr, size_t length)
+static
+void memory_discard(void *ptr, size_t length)
 {
        void *ret __attribute__((unused));
 
@@ -70,6 +120,7 @@ static void memory_discard(void *ptr, size_t length)
 
        assert(ret == ptr);
 }
+#endif /* __CYGWIN__ */
 
 static
 void cds_lfht_alloc_bucket_table(struct cds_lfht *ht, unsigned long order)