linux-kernel - Re: [PATCH] bcachefs: fix last_seq and last_empty_seq in bch2_fs_journal

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Date: Sun, 19 May 2024 22:42:52 -0400 (-04)
From: Camila Alvarez Inostroza <cam.alvarez.i@...il.com>
To: Kent Overstreet <kent.overstreet@...ux.dev>
cc: Camila Alvarez <cam.alvarez.i@...il.com>, 
    Brian Foster <bfoster@...hat.com>, linux-bcachefs@...r.kernel.org, 
    linux-kernel@...r.kernel.org, 
    syzbot+10b936c5eaee2819b49b@...kaller.appspotmail.com
Subject: Re: [PATCH] bcachefs: fix last_seq and last_empty_seq in
 bch2_fs_journal_start()



On Sun, 19 May 2024, Kent Overstreet wrote:

> 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?
>
You're right. I think the code is written in a confusing way. In 
particular it seems that cur_seq - 1 is used all over the place.
I believe we can abstract cur_seq - 1 in an independent variable (since 
it represents the actual last sequence number), that should make it 
clearer.
I'll share an updated version of the patch.
Thanks for the response!
>>
>> Reported-by: syzbot+10b936c5eaee2819b49b@...kaller.appspotmail.com
>> Signed-off-by: Camila Alvarez <cam.alvarez.i@...il.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
>>
>