bcachefs: Inode backpointers

This patch adds two new inode fields, bi_dir and bi_dir_offset, that
point back to the inode's dirent.

Since we're only adding fields for a single backpointer, files that have
been hardlinked won't necessarily have valid backpointers: we also add a
new inode flag, BCH_INODE_BACKPTR_UNTRUSTED, that's set if an inode has
ever had multiple links to it. That's ok, because we only really need
this functionality for directories, which can never have multiple
hardlinks - when we add subvolumes, we'll need a way to enemurate and
print subvolumes, and this will let us reconstruct a path to a subvolume
root given a subvolume root inode.

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

1 files changed, 43 insertions, 0 deletions

diff --git a/fs/bcachefs/fsck.c b/fs/bcachefs/fsck.c
index a3acae0ddfa9..d65b3e100f78 100644
--- a/fs/bcachefs/fsck.c
+++ b/fs/bcachefs/fsck.c
@@ -675,6 +675,39 @@ retry:
 			continue;
 		}
 
+		if (!target.bi_nlink &&
+		    !(target.bi_flags & BCH_INODE_BACKPTR_UNTRUSTED) &&
+		    (target.bi_dir != k.k->p.inode ||
+		     target.bi_dir_offset != k.k->p.offset) &&
+		    (fsck_err_on(c->sb.version >= bcachefs_metadata_version_inode_backpointers, c,
+				 "inode %llu has wrong backpointer:\n"
+				 "got       %llu:%llu\n"
+				 "should be %llu:%llu",
+				 d_inum,
+				 target.bi_dir,
+				 target.bi_dir_offset,
+				 k.k->p.inode,
+				 k.k->p.offset) ||
+		     c->opts.version_upgrade)) {
+			struct bkey_inode_buf p;
+
+			target.bi_dir		= k.k->p.inode;
+			target.bi_dir_offset	= k.k->p.offset;
+			bch2_trans_unlock(&trans);
+
+			bch2_inode_pack(c, &p, &target);
+
+			ret = bch2_btree_insert(c, BTREE_ID_inodes,
+						&p.inode.k_i, NULL, NULL,
+						BTREE_INSERT_NOFAIL|
+						BTREE_INSERT_LAZY_RW);
+			if (ret) {
+				bch_err(c, "error in fsck: error %i updating inode", ret);
+				goto err;
+			}
+			continue;
+		}
+
 		if (fsck_err_on(have_target &&
 				d.v->d_type !=
 				mode_to_type(target.bi_mode), c,
@@ -1314,6 +1347,16 @@ static int check_inode(struct btree_trans *trans,
 		do_update = true;
 	}
 
+	if (!S_ISDIR(u.bi_mode) &&
+	    u.bi_nlink &&
+	    !(u.bi_flags & BCH_INODE_BACKPTR_UNTRUSTED) &&
+	    (fsck_err_on(c->sb.version >= bcachefs_metadata_version_inode_backpointers, c,
+			 "inode missing BCH_INODE_BACKPTR_UNTRUSTED flags") ||
+	     c->opts.version_upgrade)) {
+		u.bi_flags |= BCH_INODE_BACKPTR_UNTRUSTED;
+		do_update = true;
+	}
+
 	if (do_update) {
 		struct bkey_inode_buf p;