diff --git a/tools/perf/util/mmap.c b/tools/perf/util/mmap.c
index 05076e6839382e4d098562f3c42e317d038e971a..97cf4fab564b4ff238ea0e7249a3a6b485e3b089 100644
--- a/tools/perf/util/mmap.c
+++ b/tools/perf/util/mmap.c
@@ -302,6 +302,8 @@ int perf_mmap__push(struct perf_mmap *md, bool overwrite,
 			return -1;
 	}
 
+	size = end - start;
+
 	if ((start & md->mask) + size != (end & md->mask)) {
 		buf = &data[start & md->mask];
 		size = md->mask + 1 - (start & md->mask);