Re: [PATCH] jbd2: enforce power-of-two default revoke hash size at compile time

[Andreas Dilger]

On Apr 14, 2026, at 06:59, Jan Kara <jack@xxxxxxx> wrote:
> 
> On Mon 13-04-26 14:27:24, Milos Nikic wrote:
>> The jbd2 revoke table relies on bitwise AND operations for fast hash
>> indexing, which requires the hash table size to be a strict power of two.
>> 
>> Currently, this requirement is only enforced at runtime via a J_ASSERT
>> in jbd2_journal_init_revoke(). While this successfully catches invalid
>> dynamic allocations, it means a developer accidentally modifying the
>> hardcoded JOURNAL_REVOKE_DEFAULT_HASH macro will experience a system
>> panic upon mounting the filesystem during testing.
>> 
>> Add a BUILD_BUG_ON() in journal_init_common() to validate the default
>> macro at compile time. This acts as an immediate, zero-overhead
>> safeguard, preventing compilation entirely if the default hash size is
>> mathematically invalid.
>> 
>> Signed-off-by: Milos Nikic <nikic.milos@xxxxxxxxx>
> 
> Eh, if you modify JOURNAL_REVOKE_DEFAULT_HASH you should better know what
> you are doing and if you mess up, then the kernel failing with assertion
> isn't that difficult to diagnose. So sorry I don't think this "cleanup" is
> useful either.

Jan,
this is a BUILD_BUG_ON() so it won't cause any runtime assertion.

Cheers, Andreas 

> Honza
> 
>> ---
>> fs/jbd2/journal.c | 1 +
>> 1 file changed, 1 insertion(+)
>> 
>> diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
>> index 4f397fcdb13c..62b36a2fc4e2 100644
>> --- a/fs/jbd2/journal.c
>> +++ b/fs/jbd2/journal.c
>> @@ -1565,6 +1565,7 @@ static journal_t *journal_init_common(struct 
>>  	/* The journal is marked for error until we succeed with recovery! */
>>  	journal->j_flags = JBD2_ABORT;
>> 
>> +	BUILD_BUG_ON(!is_power_of_2(JOURNAL_REVOKE_DEFAULT_HASH));
>>  	/* Set up a default-sized revoke table for the new mount. */
>>  	err = jbd2_journal_init_revoke(journal, JOURNAL_REVOKE_DEFAULT_HASH);
>>  	if (err)
>> -- 
>> 2.53.0
>> 
> -- 
> Jan Kara <jack@xxxxxxxx>
> SUSE Labs, CR
> 


Cheers, Andreas