diff --git a/storage/innobase/buf/buf0flu.cc b/storage/innobase/buf/buf0flu.cc
index ad026abf64f59..101fe6b72c368 100644
--- a/storage/innobase/buf/buf0flu.cc
+++ b/storage/innobase/buf/buf0flu.cc
@@ -2294,6 +2294,12 @@ redo log capacity filled threshold.
 @return true if adaptive flushing is recommended. */
 static bool af_needed_for_redo(lsn_t oldest_lsn) noexcept
 {
+  /* We may have oldest_lsn > log_sys.get_lsn_approx() if
+  log_t::write_buf() or log_t::persist() are executing concurrently
+  with this. In that case, age > af_lwm should hold, and
+  buf_flush_page_cleaner() would execute one more timed wait. (Not a
+  big problem.) */
+
   lsn_t age= log_sys.get_lsn_approx() - oldest_lsn;
   lsn_t af_lwm= static_cast<lsn_t>(srv_adaptive_flushing_lwm *
     static_cast<double>(log_sys.log_capacity) / 100);
@@ -2355,8 +2361,10 @@ static ulint page_cleaner_flush_pages_recommendation(ulint last_pages_in,
 	ulint			n_pages = 0;
 
 	const lsn_t cur_lsn = log_sys.get_lsn_approx();
-	ut_ad(oldest_lsn <= cur_lsn);
-	ulint pct_for_lsn = af_get_pct_for_lsn(cur_lsn - oldest_lsn);
+	/* We may have cur_lsn < oldest_lsn if
+	log_t::write_buf() or log_t::persist() were executing concurrently. */
+	ulint pct_for_lsn = cur_lsn < oldest_lsn
+                ? 0 : af_get_pct_for_lsn(cur_lsn - oldest_lsn);
 	time_t curr_time = time(nullptr);
 	const double max_pct = srv_max_buf_pool_modified_pct;
 
diff --git a/storage/innobase/include/log0log.h b/storage/innobase/include/log0log.h
index 55ffbc0df996a..134bb1f756787 100644
--- a/storage/innobase/include/log0log.h
+++ b/storage/innobase/include/log0log.h
@@ -406,9 +406,10 @@ struct log_t
   @param encrypted  whether the log is encrypted */
   static void header_write(byte *buf, lsn_t lsn, bool encrypted) noexcept;
 
-  /** @return a lower bound estimate of get_lsn(),
+  /** @return an estimate of get_lsn(),
   using acquire-release ordering with write_buf() or persist();
-  this is exact unless append_prepare_wait() is pending */
+  an upper bound if said functions have updated only one of the fields,
+  a lower bound if append_prepare_wait() is pending, otherwise exact */
   lsn_t get_lsn_approx() const noexcept
   {
     /* acquire-release ordering with write_buf() and persist() */