bcachefs: Filesystem discard option now propagates to devices

the discard option is special, because it's both a filesystem and a
device option.

When set at the filesytsem level, it's supposed to propagate to (if set
persistently via sysfs) or override (if non persistently as a mount
option) the devices - that now works correctly.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/alloc_background.c

@@ -1806,6 +1806,19 @@ struct discard_buckets_state {
 	u64		discarded;
 };
 
+/*
+ * This is needed because discard is both a filesystem option and a device
+ * option, and mount options are supposed to apply to that mount and not be
+ * persisted, i.e. if it's set as a mount option we can't propagate it to the
+ * device.
+ */
+static inline bool discard_opt_enabled(struct bch_fs *c, struct bch_dev *ca)
+{
+	return test_bit(BCH_FS_discard_mount_opt_set, &c->flags)
+		? c->opts.discard
+		: ca->mi.discard;
+}
+
 static int bch2_discard_one_bucket(struct btree_trans *trans,
 				   struct bch_dev *ca,
 				   struct btree_iter *need_discard_iter,
@@ -1869,7 +1882,7 @@ static int bch2_discard_one_bucket(struct btree_trans *trans,
 		s->discarded++;
 		*discard_pos_done = iter.pos;
 
-		if (ca->mi.discard && !c->opts.nochanges) {
+		if (discard_opt_enabled(c, ca) && !c->opts.nochanges) {
 			/*
 			 * This works without any other locks because this is the only
 			 * thread that removes items from the need_discard tree

fs/bcachefs/bcachefs.h

@@ -627,7 +627,8 @@ struct bch_dev {
 	x(topology_error)		\
 	x(errors_fixed)			\
 	x(errors_not_fixed)		\
-	x(no_invalid_checks)
+	x(no_invalid_checks)		\
+	x(discard_mount_opt_set)	\
 
 enum bch_fs_flags {
 #define x(n)		BCH_FS_##n,

fs/bcachefs/fs.c

@@ -2172,6 +2172,9 @@ static int bch2_fs_get_tree(struct fs_context *fc)
 	if (ret)
 		goto err;
 
+	if (opt_defined(opts, discard))
+		set_bit(BCH_FS_discard_mount_opt_set, &c->flags);
+
 	/* Some options can't be parsed until after the fs is started: */
 	opts = bch2_opts_empty();
 	ret = bch2_parse_mount_opts(c, &opts, NULL, opts_parse->parse_later.buf);

fs/bcachefs/sysfs.c

@@ -664,6 +664,15 @@ static ssize_t sysfs_opt_store(struct bch_fs *c,
 	    c->copygc_thread)
 		wake_up_process(c->copygc_thread);
 
+	if (id == Opt_discard && !ca) {
+		mutex_lock(&c->sb_lock);
+		for_each_member_device(c, ca)
+			opt->set_member(bch2_members_v2_get_mut(ca->disk_sb.sb, ca->dev_idx), v);
+
+		bch2_write_super(c);
+		mutex_unlock(&c->sb_lock);
+	}
+
 	ret = size;
 err:
 	up_write(&c->state_lock);