Apply arc_shrink_shift to ARC above arc_c_min

It makes sense to free memory in smaller chunks when approaching
arc_c_min to let other kernel subsystems to free more, since after
that point we can't free anything.  This also matches behavior on
Linux, where to shrinker reported only the size above arc_c_min.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Allan Jude <allan@klarasystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Closes #13794

module/os/freebsd/zfs/arc_os.c

@@ -223,7 +223,10 @@ arc_lowmem(void *arg __unused, int howto __unused)
 	arc_warm = B_TRUE;
 	arc_growtime = gethrtime() + SEC2NSEC(arc_grow_retry);
 	free_memory = arc_available_memory();
-	to_free = (arc_c >> arc_shrink_shift) - MIN(free_memory, 0);
+	int64_t can_free = arc_c - arc_c_min;
+	if (can_free <= 0)
+		return;
+	to_free = (can_free >> arc_shrink_shift) - MIN(free_memory, 0);
 	DTRACE_PROBE2(arc__needfree, int64_t, free_memory, int64_t, to_free);
 	arc_reduce_target_size(to_free);
 

module/zfs/arc.c

@@ -5036,10 +5036,11 @@ arc_reap_cb(void *arg, zthr_t *zthr)
 	 */
 	free_memory = arc_available_memory();
 
-	int64_t to_free =
-	    (arc_c >> arc_shrink_shift) - free_memory;
-	if (to_free > 0) {
-		arc_reduce_target_size(to_free);
+	int64_t can_free = arc_c - arc_c_min;
+	if (can_free > 0) {
+		int64_t to_free = (can_free >> arc_shrink_shift) - free_memory;
+		if (to_free > 0)
+			arc_reduce_target_size(to_free);
 	}
 	spl_fstrans_unmark(cookie);
 }