diff --git a/lib/libc/stdlib/malloc/mrs/mrs.c b/lib/libc/stdlib/malloc/mrs/mrs.c
index a20f76d1bd4c..cf1f129248ed 100644
--- a/lib/libc/stdlib/malloc/mrs/mrs.c
+++ b/lib/libc/stdlib/malloc/mrs/mrs.c
@@ -91,14 +91,21 @@
  *
  * Values:
  *
- * QUARANTINE_HIGHWATER: Limit the quarantine size to
- *   QUARANTINE_HIGHWATER number of bytes.
- * QUARANTINE_RATIO: Limit the quarantine size to 1 / QUARANTINE_RATIO
- *   times the size of the heap (default 4).
- * CONCURRENT_REVOCATION_PASSES: Number of concurrent revocation pass
- *   before the stop-the-world pass.
+ * QUARANTINE_NUMERATOR / QUARANTINE_DENOMINATOR: Limit the quarantine
+ * size to QUARANTINE_NUMERATOR / QUARANTINE_DENOMINATOR times the size
+ * of the heap (default 1/4).
  */
 
+#ifdef QUARANTINE_RATIO
+#error QUARANTINE_RATIO is obsolete, use QUARANTINE_NUMERATOR/QUARANTINE_DENOMINATOR
+#endif
+#ifndef QUARANTINE_DENOMINATOR
+#define	QUARANTINE_DENOMINATOR	4
+#endif
+#ifndef QUARANTINE_NUMERATOR
+#define	QUARANTINE_NUMERATOR	1
+#endif
+
 #define	MALLOCX_LG_ALIGN_BITS	6
 #define	MALLOCX_LG_ALIGN_MASK	((1 << MALLOCX_LG_ALIGN_BITS) - 1)
 /* Use MALLOCX_ALIGN_GET() if alignment may not be specified in flags. */
@@ -132,6 +139,11 @@ extern void snmalloc_flush_message_queue(void);
 #define	MALLOC_NOBOUND_CHERI_POINTERS \
 	"_RUNTIME_NOBOUND_CHERI_POINTERS"
 
+#define	MALLOC_QUARANTINE_DENOMINATOR_ENV \
+	"_RUNTIME_QUARANTINE_DENOMINATOR"
+#define	MALLOC_QUARANTINE_NUMERATOR_ENV \
+	"_RUNTIME_QUARANTINE_NUMERATOR"
+
 /*
  * Different allocators give their strong symbols different names.  Hide
  * this implementation detail being the REAL() macro.
@@ -307,6 +319,9 @@ static bool bound_pointers = false;
 static bool abort_on_validation_failure = true;
 static bool mrs_initialized = false;
 
+static unsigned int quarantine_denominator = QUARANTINE_DENOMINATOR;
+static unsigned int quarantine_numerator = QUARANTINE_NUMERATOR;
+
 static spinlock_t mrs_init_lock = _SPINLOCK_INITIALIZER;
 #define	MRS_LOCK(x)	__extension__ ({	\
 	if (__isthreaded)			\
@@ -757,20 +772,19 @@ quarantine_should_flush(struct mrs_quarantine *quarantine, bool is_free)
 		return false;
 #endif
 
-#if defined(QUARANTINE_HIGHWATER)
-
-	return (quarantine->size >= QUARANTINE_HIGHWATER);
-
-#else /* QUARANTINE_HIGHWATER */
-
-#if !defined(QUARANTINE_RATIO)
-#  define QUARANTINE_RATIO 4
-#endif /* !QUARANTINE_RATIO */
-
-	return ((allocated_size >= MIN_REVOKE_HEAP_SIZE) &&
-	    ((quarantine->size * QUARANTINE_RATIO) >= allocated_size));
+	if (allocated_size < MIN_REVOKE_HEAP_SIZE)
+		return false;
 
-#endif /* !QUARANTINE_HIGHWATER */
+	/*
+	 * Flush quarantine if
+	 *                                       quarantine_numerator
+	 * quarantine->size >= allocated_size * ----------------------
+	 *                                      quarantine_denominator
+	 *
+	 * Avoid division by multiplying both sides by quarantine_numerator.
+	 */
+	return (quarantine->size * quarantine_denominator >=
+	    allocated_size * quarantine_numerator);
 }
 
 static void
@@ -1307,6 +1321,50 @@ mrs_init_impl_locked(void)
 		exit(7);
 	}
 
+	char *envstr, *end;
+	if ((envstr = secure_getenv(MALLOC_QUARANTINE_DENOMINATOR_ENV)) !=
+	    NULL) {
+		errno = 0;
+		quarantine_denominator = strtoul(envstr, &end, 0);
+		if (*end != '\0' ||
+		    (quarantine_denominator == ULONG_MAX &&
+		     errno == ERANGE)) {
+			mrs_puts("invalid "
+			    MALLOC_QUARANTINE_DENOMINATOR_ENV "\n");
+			exit(7);
+		}
+	}
+	if ((envstr = secure_getenv(MALLOC_QUARANTINE_NUMERATOR_ENV)) !=
+	    NULL) {
+		errno = 0;
+		quarantine_numerator = strtoul(envstr, &end, 0);
+		if (*end != '\0' ||
+		    (quarantine_numerator == ULONG_MAX &&
+		     errno == ERANGE)) {
+			mrs_puts("invalid "
+			    MALLOC_QUARANTINE_NUMERATOR_ENV "\n");
+			exit(7);
+		}
+	}
+	if (quarantine_denominator == 0) {
+		mrs_puts("quarantine_denominator can not be 0\n");
+		exit(7);
+	}
+	if (quarantine_denominator > 256) {
+		/* Could overflow with 56-bits of userspace addresses */
+		mrs_puts("quarantine_denominator > 256\n");
+		exit(7);
+	}
+	if (quarantine_numerator == 0) {
+		mrs_puts("quarantine_numerator can not be 0\n");
+		exit(7);
+	}
+	if (quarantine_numerator > 256) {
+		/* Could overflow with 56-bits of userspace addresses */
+		mrs_puts("quarantine_numerator > 256\n");
+		exit(7);
+	}
+
 	if (!issetugid()) {
 		mrs_utrace = (getenv(MRS_UTRACE_ENV) != NULL);
 	}