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

[Jan Kara]

Hi!

On Wed 15-04-26 19:35:21, Andreas Dilger wrote:
> 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.

Yes, I know. But there is already a runtime assertion in
jbd2_journal_init_revoke() making sure the passed value is a power of two
which is verifying also other callers that aren't using
JOURNAL_REVOKE_DEFAULT_HASH value. I don't see a value in the *additional*
BUILD_BUG_ON when we already have that runtime check.

								Honza

-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR