[PATCH v2 1/2] use file-backed mmap on Linux

[Eric Wong <e@80x24.org>]

Due to the high memory usage of mwrap, it may be useful to force
allocations onto file-backed maps.  This means more physical
memory for the rest of the system.  As opposed to adding swap
indiscriminately, this provides the ability to give the
equivalent of swap space only to the processes using mwrap.

O_TMPFILE in Linux makes supporting this a breeze, since we
can't expect to use tmpfile(3) before malloc is initialized.
---
 mymalloc.h | 47 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 47 insertions(+)

diff --git a/mymalloc.h b/mymalloc.h
index 4904b74..88190d5 100644
--- a/mymalloc.h
+++ b/mymalloc.h
@@ -24,6 +24,49 @@
 #include <urcu/rculist.h>
 #include <urcu/wfcqueue.h>
 #include <urcu-bp.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include <errno.h>
+
+#if !defined(MWRAP_FILE_BACKED) && defined(__linux__) && defined(O_TMPFILE)
+#	define MWRAP_FILE_BACKED 1
+#else
+#	define MWRAP_FILE_BACKED 0
+#endif
+#if MWRAP_FILE_BACKED
+#	include <sys/mman.h>
+static void *my_mmap(size_t size)
+{
+	int flags = MAP_PRIVATE;
+	const char *tmpdir = getenv("TMPDIR");
+	int fd;
+	void *ret;
+
+	if (!tmpdir)
+		tmpdir = "/tmp";
+
+	fd = open(tmpdir, O_TMPFILE|O_RDWR|S_IRUSR);
+	if (fd < 0) {
+		flags |= MAP_ANONYMOUS;
+	} else {
+		ssize_t w = pwrite(fd, "", 1, size - 1); /* sparse file */
+
+		if (w <= 0) { /* 0 may be ENOSPC */
+			flags |= MAP_ANONYMOUS;
+			close(fd);
+			fd = -1;
+		}
+	}
+	ret = mmap(NULL, size, PROT_READ|PROT_WRITE, flags, fd, 0);
+	if (fd >= 0) {
+		int err = errno;
+		close(fd);
+		errno = err;
+	}
+	return ret;
+}
+#endif /* MWRAP_FILE_BACKED */
 
 /* knobs for dlmalloc */
 #define FOOTERS 1 /* required for remote_free_* stuff */
@@ -32,6 +75,10 @@
 #define DLMALLOC_EXPORT static inline
 /* #define NO_MALLOC_STATS 1 */
 #define USE_LOCKS 0 /* we do our own global_mtx + ms_tsd */
+#if MWRAP_FILE_BACKED
+#	define MMAP(size) my_mmap(size)
+#	define HAVE_MREMAP 0
+#endif
 #include "dlmalloc_c.h"
 #undef ABORT /* conflicts with Perl */