Don't ignore zfs_arc_max below allmem/32

Fixes #10157

Set arc_c_min before arc_c_max so that when zfs_arc_min is set lower
than the default allmem/32 zfs_arc_max can also be set lower.

Add warning messages when tunables are being ignored.

Signed-off-by: Ryan Moeller <ryan@iXsystems.com>

module/zfs/arc.c

@@ -6900,6 +6900,12 @@ arc_state_multilist_index_func(multilist_t *ml, void *obj)
 	    multilist_get_num_sublists(ml));
 }
 
+#define	WARN_IF_TUN_IGNORED(tun, val) do { \
+	if ((tun) && ((tun) != (val))) \
+		dprintf("WARNING: ignoring tunable %s (using %llu instead)\n",\
+		    #tun, val); \
+} while (0)
+
 /*
  * Called during module initialization and periodically thereafter to
  * apply reasonable changes to the exposed performance tunings.  Can also be
@@ -6913,6 +6919,15 @@ arc_tuning_update(void)
 	uint64_t allmem = arc_all_memory();
 	unsigned long limit;
 
+	/* Valid range: 32M - <arc_c_max> */
+	if ((zfs_arc_min) && (zfs_arc_min != arc_c_min) &&
+	    (zfs_arc_min >= 2ULL << SPA_MAXBLOCKSHIFT) &&
+	    (zfs_arc_min <= arc_c_max)) {
+		arc_c_min = zfs_arc_min;
+		arc_c = MAX(arc_c, arc_c_min);
+	}
+	WARN_IF_TUN_IGNORED(zfs_arc_min, arc_c_min);
+
 	/* Valid range: 64M - <all physical memory> */
 	if ((zfs_arc_max) && (zfs_arc_max != arc_c_max) &&
 	    (zfs_arc_max >= 64 << 20) && (zfs_arc_max < allmem) &&
@@ -6925,14 +6940,7 @@ arc_tuning_update(void)
 		if (arc_dnode_size_limit > arc_meta_limit)
 			arc_dnode_size_limit = arc_meta_limit;
 	}
-
-	/* Valid range: 32M - <arc_c_max> */
-	if ((zfs_arc_min) && (zfs_arc_min != arc_c_min) &&
-	    (zfs_arc_min >= 2ULL << SPA_MAXBLOCKSHIFT) &&
-	    (zfs_arc_min <= arc_c_max)) {
-		arc_c_min = zfs_arc_min;
-		arc_c = MAX(arc_c, arc_c_min);
-	}
+	WARN_IF_TUN_IGNORED(zfs_arc_max, arc_c_max);
 
 	/* Valid range: 16M - <arc_c_max> */
 	if ((zfs_arc_meta_min) && (zfs_arc_meta_min != arc_meta_min) &&
@@ -6944,6 +6952,7 @@ arc_tuning_update(void)
 		if (arc_dnode_size_limit < arc_meta_min)
 			arc_dnode_size_limit = arc_meta_min;
 	}
+	WARN_IF_TUN_IGNORED(zfs_arc_meta_min, arc_meta_min);
 
 	/* Valid range: <arc_meta_min> - <arc_c_max> */
 	limit = zfs_arc_meta_limit ? zfs_arc_meta_limit :
@@ -6952,6 +6961,7 @@ arc_tuning_update(void)
 	    (limit >= arc_meta_min) &&
 	    (limit <= arc_c_max))
 		arc_meta_limit = limit;
+	WARN_IF_TUN_IGNORED(zfs_arc_meta_limit, arc_meta_limit);
 
 	/* Valid range: <arc_meta_min> - <arc_meta_limit> */
 	limit = zfs_arc_dnode_limit ? zfs_arc_dnode_limit :
@@ -6960,6 +6970,7 @@ arc_tuning_update(void)
 	    (limit >= arc_meta_min) &&
 	    (limit <= arc_meta_limit))
 		arc_dnode_size_limit = limit;
+	WARN_IF_TUN_IGNORED(zfs_arc_dnode_limit, arc_dnode_size_limit);
 
 	/* Valid range: 1 - N */
 	if (zfs_arc_grow_retry)
@@ -6989,11 +7000,12 @@ arc_tuning_update(void)
 	if ((zfs_arc_lotsfree_percent >= 0) &&
 	    (zfs_arc_lotsfree_percent <= 100))
 		arc_lotsfree_percent = zfs_arc_lotsfree_percent;
+	WARN_IF_TUN_IGNORED(zfs_arc_lotsfree_percent, arc_lotsfree_percent);
 
 	/* Valid range: 0 - <all physical memory> */
 	if ((zfs_arc_sys_free) && (zfs_arc_sys_free != arc_sys_free))
 		arc_sys_free = MIN(MAX(zfs_arc_sys_free, 0), allmem);
-
+	WARN_IF_TUN_IGNORED(zfs_arc_sys_free, arc_sys_free);
 }
 
 static void