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Date: Sun, 19 May 2024 14:25:09 -0400
From: Kent Overstreet <kent.overstreet@linux.dev>
To: Camila Alvarez <cam.alvarez.i@gmail.com>
Cc: Brian Foster <bfoster@redhat.com>, linux-bcachefs@vger.kernel.org, 
	linux-kernel@vger.kernel.org, syzbot+10b936c5eaee2819b49b@syzkaller.appspotmail.com
Subject: Re: [PATCH] bcachefs: fix last_seq and last_empty_seq in
 bch2_fs_journal_start()
Message-ID: <c7qpzaeqhyvkbgaplvazj6steii5ayta7gtidmy24u7gfnnkuc@nhc2swhzmuff>
References: <20240516031919.592696-1-cam.alvarez.i@gmail.com>
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In-Reply-To: <20240516031919.592696-1-cam.alvarez.i@gmail.com>

On Wed, May 15, 2024 at 11:19:20PM -0400, Camila Alvarez wrote:
> Values were left as the next possible sequence number when there were no
> entries.
> 
> The fix involves updating the last_seq initial value and
> setting last_empty_seq to cur_seq - 1.

I think this is correct, but we should try to come up with some better
assertions or something to make the code clearer; we don't want off by
ones to lurk so easily.

Could you give it some thought?

> 
> Reported-by: syzbot+10b936c5eaee2819b49b@syzkaller.appspotmail.com
> Signed-off-by: Camila Alvarez <cam.alvarez.i@gmail.com>
> ---
>  fs/bcachefs/journal.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/bcachefs/journal.c b/fs/bcachefs/journal.c
> index adec8e1ea73e..3835c458eec9 100644
> --- a/fs/bcachefs/journal.c
> +++ b/fs/bcachefs/journal.c
> @@ -1196,7 +1196,7 @@ int bch2_fs_journal_start(struct journal *j, u64 cur_seq)
>  	struct journal_replay *i, **_i;
>  	struct genradix_iter iter;
>  	bool had_entries = false;
> -	u64 last_seq = cur_seq, nr, seq;
> +	u64 last_seq = cur_seq - 1, nr, seq;
>  
>  	genradix_for_each_reverse(&c->journal_entries, iter, _i) {
>  		i = *_i;
> @@ -1256,7 +1256,7 @@ int bch2_fs_journal_start(struct journal *j, u64 cur_seq)
>  	}
>  
>  	if (!had_entries)
> -		j->last_empty_seq = cur_seq;
> +		j->last_empty_seq = cur_seq - 1;
>  
>  	spin_lock(&j->lock);
>  
> -- 
> 2.34.1
>