diff --git a/ll-merge.c b/ll-merge.c
index 8ea03e536..4e789f533 100644
--- a/ll-merge.c
+++ b/ll-merge.c
@@ -88,7 +88,10 @@ static int ll_xdl_merge(const struct ll_merge_driver *drv_unused,
 	xmparam_t xmp;
 	assert(opts);
 
-	if (buffer_is_binary(orig->ptr, orig->size) ||
+	if (orig->size > MAX_XDIFF_SIZE ||
+	    src1->size > MAX_XDIFF_SIZE ||
+	    src2->size > MAX_XDIFF_SIZE ||
+	    buffer_is_binary(orig->ptr, orig->size) ||
 	    buffer_is_binary(src1->ptr, src1->size) ||
 	    buffer_is_binary(src2->ptr, src2->size)) {
 		return ll_binary_merge(drv_unused, result,
diff --git a/xdiff-interface.c b/xdiff-interface.c
index ecfa05f61..cb67c1c42 100644
--- a/xdiff-interface.c
+++ b/xdiff-interface.c
@@ -131,6 +131,9 @@ int xdi_diff(mmfile_t *mf1, mmfile_t *mf2, xpparam_t const *xpp, xdemitconf_t co
 	mmfile_t a = *mf1;
 	mmfile_t b = *mf2;
 
+	if (mf1->size > MAX_XDIFF_SIZE || mf2->size > MAX_XDIFF_SIZE)
+		return -1;
+
 	trim_common_tail(&a, &b, xecfg->ctxlen);
 
 	return xdl_diff(&a, &b, xpp, xecfg, xecb);
diff --git a/xdiff-interface.h b/xdiff-interface.h
index eff7762ee..fbb5a1c39 100644
--- a/xdiff-interface.h
+++ b/xdiff-interface.h
@@ -3,6 +3,13 @@
 
 #include "xdiff/xdiff.h"
 
+/*
+ * xdiff isn't equipped to handle content over a gigabyte;
+ * we make the cutoff 1GB - 1MB to give some breathing
+ * room for constant-sized additions (e.g., merge markers)
+ */
+#define MAX_XDIFF_SIZE (1024UL * 1024 * 1023)
+
 typedef void (*xdiff_emit_consume_fn)(void *, char *, unsigned long);
 
 int xdi_diff(mmfile_t *mf1, mmfile_t *mf2, xpparam_t const *xpp, xdemitconf_t const *xecfg, xdemitcb_t *ecb);