Re: [PATCH v2 2/3] ext4: avoid journaling sb update on error if journal is destroying
From: Zhang Yi
Date: Wed Mar 12 2025 - 21:20:58 EST
On 2025/3/13 1:15, Jan Kara wrote:
> On Wed 12-03-25 19:56:36, Ojaswin Mujoo wrote:
>> On Wed, Mar 12, 2025 at 11:51:03AM +0100, Jan Kara wrote:
>>> On Mon 10-03-25 10:13:36, Ritesh Harjani wrote:
>>>> Ojaswin Mujoo <ojaswin@xxxxxxxxxxxxx> writes:
>>>>> On Sun, Mar 09, 2025 at 12:11:22AM +0530, Ritesh Harjani wrote:
>>>>>> Ojaswin Mujoo <ojaswin@xxxxxxxxxxxxx> writes:
>>>>>>> On Sat, Mar 08, 2025 at 06:56:23PM +0530, Ritesh Harjani wrote:
>>>>>>>> Ojaswin Mujoo <ojaswin@xxxxxxxxxxxxx> writes:
>>>>>>>>> On Sat, Mar 08, 2025 at 03:25:04PM +0530, Ritesh Harjani (IBM) wrote:
>>>>>>>>>> Ojaswin Mujoo <ojaswin@xxxxxxxxxxxxx> writes:
>>>>>>>>>>> Presently we always BUG_ON if trying to start a transaction on a journal marked
>>>>>>>>>>> with JBD2_UNMOUNT, since this should never happen. However, while ltp running
>>>>>>>>>>> stress tests, it was observed that in case of some error handling paths, it is
>>>>>>>>>>> possible for update_super_work to start a transaction after the journal is
>>>>>>>>>>> destroyed eg:
>>>>>>>>>>>
>>>>>>>>>>> (umount)
>>>>>>>>>>> ext4_kill_sb
>>>>>>>>>>> kill_block_super
>>>>>>>>>>> generic_shutdown_super
>>>>>>>>>>> sync_filesystem /* commits all txns */
>>>>>>>>>>> evict_inodes
>>>>>>>>>>> /* might start a new txn */
>>>>>>>>>>> ext4_put_super
>>>>>>>>>>> flush_work(&sbi->s_sb_upd_work) /* flush the workqueue */
>>>>>>>>>>> jbd2_journal_destroy
>>>>>>>>>>> journal_kill_thread
>>>>>>>>>>> journal->j_flags |= JBD2_UNMOUNT;
>>>>>>>>>>> jbd2_journal_commit_transaction
>>>>>>>>>>> jbd2_journal_get_descriptor_buffer
>>>>>>>>>>> jbd2_journal_bmap
>>>>>>>>>>> ext4_journal_bmap
>>>>>>>>>>> ext4_map_blocks
>>>>>>>>>>> ...
>>>>>>>>>>> ext4_inode_error
>>>>>>>>>>> ext4_handle_error
>>>>>>>>>>> schedule_work(&sbi->s_sb_upd_work)
>>>>>>>>>>>
>>>>>>>>>>> /* work queue kicks in */
>>>>>>>>>>> update_super_work
>>>>>>>>>>> jbd2_journal_start
>>>>>>>>>>> start_this_handle
>>>>>>>>>>> BUG_ON(journal->j_flags &
>>>>>>>>>>> JBD2_UNMOUNT)
>>>>>>>>>>>
>>>>>>>>>>> Hence, introduce a new sbi flag s_journal_destroying to indicate journal is
>>>>>>>>>>> destroying only do a journaled (and deferred) update of sb if this flag is not
>>>>>>>>>>> set. Otherwise, just fallback to an un-journaled commit.
>>>>>>>>>>>
>>>>>>>>>>> We set sbi->s_journal_destroying = true only after all the FS updates are done
>>>>>>>>>>> during ext4_put_super() (except a running transaction that will get commited
>>>>>>>>>>> during jbd2_journal_destroy()). After this point, it is safe to commit the sb
>>>>>>>>>>> outside the journal as it won't race with a journaled update (refer
>>>>>>>>>>> 2d01ddc86606).
>>>>>>>>>>>
>>>>>>>>>>> Also, we don't need a similar check in ext4_grp_locked_error since it is only
>>>>>>>>>>> called from mballoc and AFAICT it would be always valid to schedule work here.
>>>>>>>>>>>
>>>>>>>>>>> Fixes: 2d01ddc86606 ("ext4: save error info to sb through journal if available")
>>>>>>>>>>> Reported-by: Mahesh Kumar <maheshkumar657g@xxxxxxxxx>
>>>>>>>>>>> Suggested-by: Jan Kara <jack@xxxxxxx>
>>>>>>>>>>> Signed-off-by: Ojaswin Mujoo <ojaswin@xxxxxxxxxxxxx>
>>>>>>>>>>> ---
>>>>>>>>>>> fs/ext4/ext4.h | 2 ++
>>>>>>>>>>> fs/ext4/ext4_jbd2.h | 8 ++++++++
>>>>>>>>>>> fs/ext4/super.c | 4 +++-
>>>>>>>>>>> 3 files changed, 13 insertions(+), 1 deletion(-)
>>>>>>>>>>>
>>>>>>>>>>> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
>>>>>>>>>>> index 2b7d781bfcad..d48e93bd5690 100644
>>>>>>>>>>> --- a/fs/ext4/ext4.h
>>>>>>>>>>> +++ b/fs/ext4/ext4.h
>>>>>>>>>>> @@ -1728,6 +1728,8 @@ struct ext4_sb_info {
>>>>>>>>>>> */
>>>>>>>>>>> struct work_struct s_sb_upd_work;
>>>>>>>>>>>
>>>>>>>>>>> + bool s_journal_destorying;
>>>>>>>>>>> +
>>>>>>>>>>> /* Atomic write unit values in bytes */
>>>>>>>>>>> unsigned int s_awu_min;
>>>>>>>>>>> unsigned int s_awu_max;
>>>>>>>>>>> diff --git a/fs/ext4/ext4_jbd2.h b/fs/ext4/ext4_jbd2.h
>>>>>>>>>>> index 9b3c9df02a39..6bd3ca84410d 100644
>>>>>>>>>>> --- a/fs/ext4/ext4_jbd2.h
>>>>>>>>>>> +++ b/fs/ext4/ext4_jbd2.h
>>>>>>>>>>> @@ -437,6 +437,14 @@ static inline int ext4_journal_destroy(struct ext4_sb_info *sbi, journal_t *jour
>>>>>>>>>>> {
>>>>>>>>>>> int err = 0;
>>>>>>>>>>>
>>>>>>>>>>> + /*
>>>>>>>>>>> + * At this point all pending FS updates should be done except a possible
>>>>>>>>>>> + * running transaction (which will commit in jbd2_journal_destroy). It
>>>>>>>>>>> + * is now safe for any new errors to directly commit superblock rather
>>>>>>>>>>> + * than going via journal.
>>>>>>>>>>> + */
>>>>>>>>>>> + sbi->s_journal_destorying = true;
>>>>>>>>>>
>>>>>>>>>> This is not correct right. I think what we decided to set this flag
>>>>>>>>>> before we flush the workqueue. So that we don't schedule any new
>>>>>>>>>> work after this flag has been set. At least that is what I understood.
>>>>>>>>>>
>>>>>>>>>> [1]: https://lore.kernel.org/all/87eczc6rlt.fsf@xxxxxxxxx/
>>>>>>>>>>
>>>>>>>>>> -ritesh
>>>>>>>>>
>>>>>>>>> Hey Ritesh,
>>>>>>>>>
>>>>>>>>> Yes that is not correct, I missed that in my patch however we realised
>>>>>>>>> that adding it before flush_work() also has issues [1]. More
>>>>>>>>> specifically:
>>>>>>>>
>>>>>>>> Ohk. right.
>>>>>>>>
>>>>>>>>>
>>>>>>>>> **kjournald2**
>>>>>>>>> jbd2_journal_commit_transaction()
>>>>>>>>> ...
>>>>>>>>> ext4_handle_error()
>>>>>>>>> /* s_journal_destorying is not set */
>>>>>>>>> if (journal && !s_journal_destorying)
>>>>>>>>
>>>>>>>> Then maybe we should not schedule another work to update the superblock
>>>>>>>> via journalling, it the error itself occurred while were trying to
>>>>>>>> commit the journal txn?
>>>>>>>>
>>>>>>>>
>>>>>>>> -ritesh
>>>>>>>
>>>>>>> Hmm, ideally yes that should not happen, but how can we achieve that?
>>>>>>> For example with the trace we saw:
>>>>>>>
>>>>>>> **kjournald2**
>>>>>>> jbd2_journal_commit_transaction()
>>>>>>> jbd2_journal_get_descriptor_buffer
>>>>>>> jbd2_journal_bmap
>>>>>>> ext4_journal_bmap
>>>>>>> ext4_map_blocks
>>>>>>> ...
>>>>>>> ext4_inode_error
>>>>>>> ext4_handle_error
>>>>>>> schedule_work(&sbi->s_sb_upd_work)
>>>>>>>
>>>>>>> How do we tell ext4_handle_error that it is in the context of a
>>>>>>> committing txn.
>>>
>>> So I was thinking about this. It is not a problem to determine we are
>>> running in kjournald context - it is enough to check
>>>
>>> current == EXT4_SB(sb)->s_journal->j_task
>>
>> Oh, right :)
>>
>>>
>>> But I'm not sure checking this in ext4_handle_error() and doing direct sb
>>> update instead of scheduling a journalled one is always correct. For
>>> example kjournald does also writeback of ordered data and if that hits an
>>> error, we do not necessarily abort the journal (well, currently we do as
>>> far as I'm checking but it seems a bit fragile to rely on this).
>>
>> Okay so IIUC your concern is there might be some codepaths, now or in
>> the future, where kjournald might call the FS layer, hit an error and
>> still decide to not abort. In which case we would still want to update
>> the sb via journal.
>
> Yeah. The reason why I'm a bit concerned about it is mostly the case of
> kjournald also handling ordered data and situations like
> !(journal->j_flags & JBD2_ABORT_ON_SYNCDATA_ERR) where people want to
> continue although ordered data had issues. Or situations where something in
> j_commit_callback or another jbd2 hook ends up calling ext4_error()...
>
Ha, right! This is a case where kjournald triggers an ext4 error but does
not abort the journal for now, I forgot this one, and there may be more.
Thanks for pointing it out. I would also prefer to use this solution of
adding ext4_journal_destory().
Thanks,
Yi.