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

[Milos Nikic]

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>
---
 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 block_device *bdev,
 	/* 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