bcachefs: Fix subvol to missing root repairbcachefs-for-6.15-2025-06-05bcachefs-for-v6.15

We had a bug where the root inode of a subvolume was erronously deleted:
bch2_evict_inode() called bch2_inode_rm(), meaning the VFS inode's
i_nlink was somehow set to 0 when it shouldn't have - the inode in the
btree indicated it clearly was not unlinked.

This has been addressed with additional safety checks in
bch2_inode_rm() - pulling in the safety checks we already were doing
when deleting unlinked inodes in recovery - but the really disastrous
bug was in check_subvols(), which on finding a dangling subvol (subvol
with a missing root inode) would delete the subvolume.

I assume this bug dates from early check_directory_structure() code,
which originally handled subvolumes and normal paths - the idea being
that still live contents of the subvolume would get reattached
somewhere.

But that's incorrect, and disastrously so; deleting a subvolume triggers
deleting the snapshot ID it points to, deleting the entire contents.

The correct way to repair is to recreate the root inode if it's missing;
then any contents will get reattached under that subvolume's lost+found.

(cherry picked from commit 29cc6fb7c068c773049d3bde14b939033893eff4)
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

1 files changed, 15 insertions, 4 deletions

diff --git a/fs/bcachefs/subvolume.c b/fs/bcachefs/subvolume.c
index d0209f7658bb..bc6009a71284 100644
--- a/fs/bcachefs/subvolume.c
+++ b/fs/bcachefs/subvolume.c
@@ -6,6 +6,7 @@
 #include "errcode.h"
 #include "error.h"
 #include "fs.h"
+#include "inode.h"
 #include "recovery_passes.h"
 #include "snapshot.h"
 #include "subvolume.h"
@@ -113,10 +114,20 @@ static int check_subvol(struct btree_trans *trans,
 			     "subvolume %llu points to missing subvolume root %llu:%u",
 			     k.k->p.offset, le64_to_cpu(subvol.v->inode),
 			     le32_to_cpu(subvol.v->snapshot))) {
-			ret = bch2_subvolume_delete(trans, iter->pos.offset);
-			bch_err_msg(c, ret, "deleting subvolume %llu", iter->pos.offset);
-			ret = ret ?: -BCH_ERR_transaction_restart_nested;
-			goto err;
+			/*
+			 * Recreate - any contents that are still disconnected
+			 * will then get reattached under lost+found
+			 */
+			bch2_inode_init_early(c, &inode);
+			bch2_inode_init_late(&inode, bch2_current_time(c),
+					     0, 0, S_IFDIR|0700, 0, NULL);
+			inode.bi_inum			= le64_to_cpu(subvol.v->inode);
+			inode.bi_snapshot		= le32_to_cpu(subvol.v->snapshot);
+			inode.bi_subvol			= k.k->p.offset;
+			inode.bi_parent_subvol		= le32_to_cpu(subvol.v->fs_path_parent);
+			ret = __bch2_fsck_write_inode(trans, &inode);
+			if (ret)
+				goto err;
 		}
 	} else {
 		goto err;