linux-kernel - Re: Sub volumes handling in bch2_fsck_update

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Message-ID: <r7uir3b5ikbzaoaeh4hmuua73kaa2ydetnhfbxc6554wjdyr6l@pkpgzp2zonan>
Date: Wed, 23 Apr 2025 13:46:27 -0400
From: Kent Overstreet <kent.overstreet@...ux.dev>
To: Arnaud Lecomte <contact@...aud-lcm.com>
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

On Wed, Apr 23, 2025 at 07:18:34PM +0200, Arnaud Lecomte wrote:
> 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.

go for it, get ktest going and join the IRC channel

https://evilpiepirate.org/git/ktest.git/

> 
> 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.
> >