Re: [PATCH 4/4] ext4: fix incorrect tid assumption in ext4_fc_mark_ineligible()

Luis Henriques
2024-07-24T14:02:59Z

On Wed, Jul 24 2024, Jan Kara wrote:

> On Tue 23-07-24 16:44:02, Luis Henriques (SUSE) wrote:
>> Function jbd2_journal_shrink_checkpoint_list() assumes that '0' is not a
>> valid value for transaction IDs, which is incorrect.
>>
>> Furthermore, the sbi->s_fc_ineligible_tid handling also makes the same
>> assumption by being initialised to '0'. Fortunately, the sb flag
>> EXT4_MF_FC_INELIGIBLE can be used to check whether sbi->s_fc_ineligible_tid
>> has been previously set instead of comparing it with '0'.
>>
>> Signed-off-by: Luis Henriques (SUSE) <luis.henriques@linux.dev>
>
> Just one style nit below, otherwise looks good. Feel free to add:
>
> Reviewed-by: Jan Kara <jack@suse.cz>
>
> BTW, the ineligibility handling looks flaky to me, in particular the cases
> where we call ext4_fc_mark_ineligible() with NULL handle seem racy to me as
> fastcommit can happen *before* we mark the filesystem as ineligible. But
> that's not really related to your changes, they just made me look at that
> code in detail and I couldn't resist complaining :)

Heh, fair enough. Regarding this race, I may try to look into it but I'll
need to dig a bit more. And yeah it's probably better to separate that
from this patch.

>
>> ---
>> fs/ext4/fast_commit.c | 15 +++++++++++----
>> 1 file changed, 11 insertions(+), 4 deletions(-)
>>
>> diff --git a/fs/ext4/fast_commit.c b/fs/ext4/fast_commit.c
>> index 3926a05eceee..3e0793cfea38 100644
>> --- a/fs/ext4/fast_commit.c
>> +++ b/fs/ext4/fast_commit.c
>> @@ -339,22 +339,29 @@ void ext4_fc_mark_ineligible(struct super_block *sb, int reason, handle_t *handl
>> {
>> struct ext4_sb_info *sbi = EXT4_SB(sb);
>> tid_t tid;
>> + bool has_transaction = true;
>> + bool is_ineligible;
>>
>> if (ext4_fc_disabled(sb))
>> return;
>>
>> - ext4_set_mount_flag(sb, EXT4_MF_FC_INELIGIBLE);
>> if (handle && !IS_ERR(handle))
>> tid = handle->h_transaction->t_tid;
>> else {
>> read_lock(&sbi->s_journal->j_state_lock);
>> - tid = sbi->s_journal->j_running_transaction ?
>> - sbi->s_journal->j_running_transaction->t_tid : 0;
>> + if (sbi->s_journal->j_running_transaction)
>> + tid = sbi->s_journal->j_running_transaction->t_tid;
>> + else
>> + has_transaction = false;
>> read_unlock(&sbi->s_journal->j_state_lock);
>> }
>> spin_lock(&sbi->s_fc_lock);
>> - if (tid_gt(tid, sbi->s_fc_ineligible_tid))
>> + is_ineligible = ext4_test_mount_flag(sb, EXT4_MF_FC_INELIGIBLE);
>> + if (has_transaction &&
>> + ((!is_ineligible) ||
> ^^ these extra braces look strange
>

They do, indeed. I think my initial version had an explicit comparison
with 'false'. v2 will have those removed. And once again, thanks for
your review, Jan!

Cheers,
--
Luís

>> + (is_ineligible && tid_gt(tid, sbi->s_fc_ineligible_tid))))
>> sbi->s_fc_ineligible_tid = tid;
>> + ext4_set_mount_flag(sb, EXT4_MF_FC_INELIGIBLE);
>
> Honza
> --
> Jan Kara <jack@suse.com>
> SUSE Labs, CR