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Message-ID: <87ecz7hcw0.fsf@gmail.com>
Date: Sat, 08 Mar 2025 18:56:23 +0530
From: Ritesh Harjani (IBM) <ritesh.list@...il.com>
To: Ojaswin Mujoo <ojaswin@...ux.ibm.com>
Cc: linux-ext4@...r.kernel.org, Theodore Ts'o <tytso@....edu>, Jan Kara <jack@...e.cz>, Baokun Li <libaokun1@...wei.com>, linux-kernel@...r.kernel.org, Mahesh Kumar <maheshkumar657g@...il.com>
Subject: Re: [PATCH v2 2/3] ext4: avoid journaling sb update on error if journal is destroying
Ojaswin Mujoo <ojaswin@...ux.ibm.com> writes:
> On Sat, Mar 08, 2025 at 03:25:04PM +0530, Ritesh Harjani (IBM) wrote:
>> Ojaswin Mujoo <ojaswin@...ux.ibm.com> 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@...il.com>
>> > Suggested-by: Jan Kara <jack@...e.cz>
>> > Signed-off-by: Ojaswin Mujoo <ojaswin@...ux.ibm.com>
>> > ---
>> > 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@gmail.com/
>>
>> -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
> ext4_put_super()
> sbi->s_journal_destorying = true;
> flush_work(&sbi->s_sb_upd_work)
> schedule_work()
> jbd2_journal_destroy()
> journal->j_flags |= JBD2_UNMOUNT;
>
> jbd2_journal_start()
> start_this_handle()
> BUG_ON(JBD2_UNMOUNT)
>
> So the right thing to do seems to be that we need to force a journal
> commit before the final flush as well. [1] Has more info on this and
> some followup discussion as well.
>
> [1] https://lore.kernel.org/all/cover.1741270780.git.ojaswin@linux.ibm.com/T/#mc8046d47b357665bdbd2878c91e51eb660f94b3e
>
> Regards,
> ojaswin
>>
>>
>> > +
>> > err = jbd2_journal_destroy(journal);
>> > sbi->s_journal = NULL;
>> >
>> > diff --git a/fs/ext4/super.c b/fs/ext4/super.c
>> > index 8ad664d47806..31552cf0519a 100644
>> > --- a/fs/ext4/super.c
>> > +++ b/fs/ext4/super.c
>> > @@ -706,7 +706,7 @@ static void ext4_handle_error(struct super_block *sb, bool force_ro, int error,
>> > * constraints, it may not be safe to do it right here so we
>> > * defer superblock flushing to a workqueue.
>> > */
>> > - if (continue_fs && journal)
>> > + if (continue_fs && journal && !EXT4_SB(sb)->s_journal_destorying)
>> > schedule_work(&EXT4_SB(sb)->s_sb_upd_work);
>> > else
>> > ext4_commit_super(sb);
>> > @@ -5311,6 +5311,8 @@ static int __ext4_fill_super(struct fs_context *fc, struct super_block *sb)
>> > spin_lock_init(&sbi->s_error_lock);
>> > INIT_WORK(&sbi->s_sb_upd_work, update_super_work);
>> >
>> > + sbi->s_journal_destorying = false;
>> > +
>> > err = ext4_group_desc_init(sb, es, logical_sb_block, &first_not_zeroed);
>> > if (err)
>> > goto failed_mount3;
>> > --
>> > 2.48.1
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