linux-kernel - Re: Sub volumes handling in bch2_fsck_update

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Message-ID: <53206807-b595-4fec-8113-a81a24c3e8db@arnaud-lcm.com>
Date: Wed, 23 Apr 2025 19:18:34 +0200
From: Arnaud Lecomte <contact@...aud-lcm.com>
To: Kent Overstreet <kent.overstreet@...ux.dev>
Cc: syzbot+843981bb836d699c07d1@...kaller.appspotmail.com,
 linux-bcachefs@...r.kernel.org, linux-kernel@...r.kernel.org,
 syzkaller-bugs@...glegroups.com
Subject: Re: Sub volumes handling in bch2_fsck_update_backpointers

Would you be  okay if I try to handle it ? I am fairly new to bcachefs 
but I am really interested to get involve into it, I like the project.

On 23/04/2025 18:47, Kent Overstreet wrote:
> On Wed, Apr 23, 2025 at 05:45:31PM +0200, Arnaud Lecomte wrote:
>> Hey everyone, in fsck.c, we have:
>> /*
>>   * Prefer to delete the first one, since that will be the one at the wrong
>>   * offset:
>>   * return value: 0 -> delete k1, 1 -> delete k2
>>   */
>> int bch2_fsck_update_backpointers(struct btree_trans *trans,
>> 				  struct snapshots_seen *s,
>> 				  const struct bch_hash_desc desc,
>> 				  struct bch_hash_info *hash_info,
>> 				  struct bkey_i *new)
>> {
>> 	if (new->k.type != KEY_TYPE_dirent)
>> 		return 0;
>>
>> 	struct bkey_i_dirent *d = bkey_i_to_dirent(new);
>> 	struct inode_walker target = inode_walker_init();
>> 	int ret = 0;
>>
>> 	if (d->v.d_type == DT_SUBVOL) {
>> 		BUG();
>> 	} else {
>> 		ret = get_visible_inodes(trans, &target, s, le64_to_cpu(d->v.d_inum));
>> 		if (ret)
>> 			goto err;
>>
>> 		darray_for_each(target.inodes, i) {
>> 			i->inode.bi_dir_offset = d->k.p.offset;
>> 			ret = __bch2_fsck_write_inode(trans, &i->inode);
>> 			if (ret)
>> 				goto err;
>> 		}
>> 	}
>> err:
>> 	inode_walker_exit(&target);
>> 	return ret;
>> }
>>
>> What is the current state for handling subvolumes ? In someone already working on or it is something we don't want to implement
>> for some reasons ?
> This does need to be handled, I haven't started on it yet.
>
> I did just fix another subvolume root backpointers bug, which makes this
> one easier - now, only the newest snapshot version of a subvolume root
> inode needs to have a backpointer.
>