diff --git a/include/linux/bug.h b/include/linux/bug.h
index a64a309c..eb53f39c 100644
--- a/include/linux/bug.h
+++ b/include/linux/bug.h
@@ -3,6 +3,7 @@
 
 #include <assert.h>
 #include <linux/compiler.h>
+#include <valgrind/memcheck.h>
 
 #define BUILD_BUG_ON_NOT_POWER_OF_2(n)			\
 	BUILD_BUG_ON((n) == 0 || (((n) & ((n) - 1)) != 0))
@@ -52,4 +53,6 @@
 	__ret_warn_on;							\
 })
 
+#define DEBUG_MEMORY_FREED(p, len) VALGRIND_MAKE_MEM_UNDEFINED(p, len)
+
 #endif /* __TOOLS_LINUX_BUG_H */
diff --git a/libbcachefs/journal_io.c b/libbcachefs/journal_io.c
index 387377da..a6a4dda5 100644
--- a/libbcachefs/journal_io.c
+++ b/libbcachefs/journal_io.c
@@ -934,6 +934,8 @@ out:
 	/* also must come before signalling write completion: */
 	closure_debug_destroy(cl);
 
+	DEBUG_MEMORY_FREED(w->data, w->buf_size);
+
 	BUG_ON(!j->reservations.prev_buf_unwritten);
 	atomic64_sub(((union journal_res_state) { .prev_buf_unwritten = 1 }).v,
 		     &j->reservations.counter);