Improve flushing pagecache on darwin

configure

@@ -2997,7 +2997,9 @@ fi
 print_config "timerfd_create" "$timerfd_create"
 
 #############################################################################
-
+if test "$targetos" = "Darwin"; then
+    output_sym "CONFIG_MAC_FADVISE_DISCARD"
+fi
 if test "$wordsize" = "64" ; then
   output_sym "CONFIG_64BIT"
 elif test "$wordsize" = "32" ; then

helpers.c

@@ -1,4 +1,7 @@
 #include <errno.h>
+#include <sys/stat.h>
+#include <sys/mman.h>
+#include <stdio.h>
 
 #include "helpers.h"
 
@@ -26,9 +29,68 @@ int sync_file_range(int fd, uint64_t offset, uint64_t nbytes,
 }
 #endif
 
+#ifdef CONFIG_MAC_FADVISE_DISCARD
+/**
+ * CONFIG_MAC_FADVISE_DISCARD should not be run within Rosetta. Native arm64 or x86 only.
+ */
+
+#define MMAP_CHUNK_SIZE		(1024 * 1024 * 1024)
+
+int discard_pages(int fd, off_t offset, off_t size)
+{
+	caddr_t *addr;
+	uint64_t chunk_size = MMAP_CHUNK_SIZE;
+
+	printf("fsync'ing file, then invalidating UBC.\n");
+	if (fsync(fd) < 0) {
+		fprintf(stderr, "Cannot fsync file\n");
+		exit(1);
+	}
+
+	/*
+	 * mmap the file in 1GB chunks and msync(MS_INVALIDATE).
+	 */
+	while (size > 0) {
+		uint64_t mmap_size = MIN(chunk_size, size);
+
+		addr = mmap((caddr_t)0, mmap_size, PROT_NONE, MAP_SHARED, fd, offset);
+		if (addr == MAP_FAILED) {
+			fprintf(stderr, "Failed to mmap (%s), offset = %llu, size = %llu\n",
+				strerror(errno), offset, mmap_size);
+			return -1;
+		}
+
+		if (msync(addr, mmap_size, MS_INVALIDATE)) {
+			fprintf(stderr, "msync failed to free cache pages.\n");
+			return -1;
+		}
+
+		/* Destroy the above mappings used to invalidate cache - cleaning up */
+		if (munmap(addr, mmap_size) < 0) {
+			fprintf(stderr, "munmap failed, error = %d.\n", errno);
+			return -1;
+		}
+
+		size -= mmap_size;
+		offset += mmap_size;
+	}
+
+	return 0;
+}
+
+int posix_fadvise(int fd, off_t offset, off_t len, int advice)
+{
+	int ret = 0;
+
+	if (advice == POSIX_FADV_DONTNEED)
+		ret = discard_pages(fd, offset, len);
+	return ret;
+}
+#else
 #ifndef CONFIG_POSIX_FADVISE
 int posix_fadvise(int fd, off_t offset, off_t len, int advice)
 {
 	return 0;
 }
 #endif
+#endif //CONFIG_MAC_FADVISE_DISCARD