diff --git a/usr.bin/diff/diffreg.c b/usr.bin/diff/diffreg.c
index 4c921151045..4c17bef4c3e 100644
--- a/usr.bin/diff/diffreg.c
+++ b/usr.bin/diff/diffreg.c
@@ -71,6 +71,7 @@ __FBSDID("$FreeBSD$");
 
 #include <sys/capsicum.h>
 #include <sys/stat.h>
+#include <sys/mman.h>
 
 #include <capsicum_helpers.h>
 #include <ctype.h>
@@ -341,7 +342,7 @@ diffreg(char *file1, char *file2, int flags, int capsicum)
 		pr = start_pr(file1, file2);
 
 	if (capsicum) {
-		cap_rights_init(&rights_ro, CAP_READ, CAP_FSTAT, CAP_SEEK);
+		cap_rights_init(&rights_ro, CAP_READ, CAP_FSTAT, CAP_SEEK, CAP_MMAP);
 		if (caph_rights_limit(fileno(f1), &rights_ro) < 0)
 			err(2, "unable to limit rights on: %s", file1);
 		if (caph_rights_limit(fileno(f2), &rights_ro) < 0)
@@ -360,6 +361,15 @@ diffreg(char *file1, char *file2, int flags, int capsicum)
 		if (caph_enter() < 0)
 			err(2, "unable to enter capability mode");
 	}
+	char *buf1 = mmap(NULL, stb1.st_size, PROT_READ, MAP_SHARED, fileno(f1), 0);
+	if (buf1 == NULL)
+		err(2, "unable to mmap");
+	msync(buf1, stb1.st_size, MS_INVALIDATE);
+	char *buf2 = mmap(NULL, stb2.st_size, PROT_READ, MAP_SHARED, fileno(f2), 0);
+	if (buf2 == NULL)
+		err(2, "unable to mmap");
+	msync(buf2, stb2.st_size, MS_INVALIDATE);
+
 
 	switch (files_differ(f1, f2, flags)) {
 	case 0: