diff --git a/NEWS b/NEWS
index 4ac67e08..942d2c20 100644
--- a/NEWS
+++ b/NEWS
@@ -15,6 +15,7 @@ NEWS
   * [autoconf] define HAVE_CRYPT when crypt() is present
   * [bsd xattr] fix compile break with BSD extended attributes in stat_cache
   * [mod_cgi] rewrite mmap and generic (post body) send error handling
+  * [mmap] fix mmap alignment
 
 - 1.4.36 - 2015-07-26
   * use keep-alive timeout while waiting for HTTP headers; use always the read timeout while waiting for the HTTP body
diff --git a/src/mod_cgi.c b/src/mod_cgi.c
index d1f4e09b..50bbc485 100644
--- a/src/mod_cgi.c
+++ b/src/mod_cgi.c
@@ -766,8 +766,8 @@ static int cgi_write_file_chunk_mmap(server *srv, connection *con, int fd, chunk
 			c->file.mmap.start = MAP_FAILED;
 		}
 
-		c->file.mmap.offset = offset;
-		c->file.mmap.length = toSend;
+		c->file.mmap.offset = mmap_align_offset(offset);
+		c->file.mmap.length = file_end - c->file.mmap.offset;
 
 		if (MAP_FAILED == (c->file.mmap.start = mmap(NULL, c->file.mmap.length, PROT_READ, MAP_SHARED, c->file.fd, c->file.mmap.offset))) {
 			log_error_write(srv, __FILE__, __LINE__, "ssbdoo", "mmap failed:",
diff --git a/src/network_backends.h b/src/network_backends.h
index 2245c749..8233c488 100644
--- a/src/network_backends.h
+++ b/src/network_backends.h
@@ -91,6 +91,7 @@ static inline int network_writev_mem_chunks(server *srv, connection *con, int fd
 /* next chunk must be FILE_CHUNK. use temporary buffer (srv->tmp_buf) to read into, then write()/send() it */
 int network_write_file_chunk_no_mmap(server *srv, connection *con, int fd, chunkqueue *cq, off_t *p_max_bytes);
 
+off_t mmap_align_offset(off_t start);
 #if defined(USE_MMAP)
 /* next chunk must be FILE_CHUNK. send mmap()ed file with write() */
 int network_write_file_chunk_mmap(server *srv, connection *con, int fd, chunkqueue *cq, off_t *p_max_bytes);
diff --git a/src/network_write_mmap.c b/src/network_write_mmap.c
index 9747d619..9ff3ae96 100644
--- a/src/network_write_mmap.c
+++ b/src/network_write_mmap.c
@@ -1,7 +1,5 @@
 #include "network_backends.h"
 
-#if defined(USE_MMAP)
-
 #include "network.h"
 #include "log.h"
 #include "sys-mmap.h"
@@ -11,6 +9,20 @@
 #include <errno.h>
 #include <string.h>
 
+#define MMAP_CHUNK_SIZE (512*1024)
+
+off_t mmap_align_offset(off_t start) {
+	static long pagesize = 0;
+	if (0 == pagesize) {
+		pagesize = sysconf(_SC_PAGESIZE);
+		force_assert(pagesize < MMAP_CHUNK_SIZE);
+	}
+	force_assert(start >= (start % pagesize));
+	return start - (start % pagesize);
+}
+
+#if defined(USE_MMAP)
+
 #if 0
 /* read mmap()ed data into local buffer */
 #define LOCAL_BUFFERING 1
@@ -70,10 +82,10 @@ int network_write_file_chunk_mmap(server *srv, connection *con, int fd, chunkque
 		 *     3. use non-blocking IO for file-transfers
 		 *   */
 
-		c->file.mmap.offset = offset & ~(4095); /* align at 4kb */
+		c->file.mmap.offset = mmap_align_offset(offset);
 
-		/* all mmap()ed areas are 512kb except the last which might be smaller */
-		c->file.mmap.length = 512*1024;
+		/* all mmap()ed areas are MMAP_CHUNK_SIZE except the last which might be smaller */
+		c->file.mmap.length = MMAP_CHUNK_SIZE;
 		if (c->file.mmap.offset > file_end - (off_t)c->file.mmap.length) {
 			c->file.mmap.length = file_end - c->file.mmap.offset;
 		}