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Message-ID: <Z8qTciy49b7LSHqr@li-dc0c254c-257c-11b2-a85c-98b6c1322444.ibm.com>
Date: Fri, 7 Mar 2025 12:04:26 +0530
From: Ojaswin Mujoo <ojaswin@...ux.ibm.com>
To: Zhang Yi <yi.zhang@...weicloud.com>
Cc: Jan Kara <jack@...e.cz>, Baokun Li <libaokun1@...wei.com>,
linux-kernel@...r.kernel.org, Mahesh Kumar <maheshkumar657g@...il.com>,
linux-ext4@...r.kernel.org, "Theodore Ts'o" <tytso@....edu>
Subject: Re: [PATCH v2 2/3] ext4: avoid journaling sb update on error if
journal is destroying
On Fri, Mar 07, 2025 at 10:49:28AM +0800, Zhang Yi wrote:
> On 2025/3/6 22:28, Ojaswin Mujoo wrote:
> > 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;
> > +
>
> Hi, Ojaswin!
>
> I'm afraid you still need to flush the superblock update work here,
> otherwise I guess the race condition you mentioned in v1 could still
> occur.
>
> ext4_put_super()
> flush_work(&sbi->s_sb_upd_work)
>
> **kjournald2**
> jbd2_journal_commit_transaction()
> ...
> ext4_inode_error()
> /* JBD2_UNMOUNT not set */
> schedule_work(s_sb_upd_work)
>
> **workqueue**
> update_super_work
> /* s_journal_destorying is not set */
> if (journal && !s_journal_destorying)
>
> ext4_journal_destroy()
> /* set s_journal_destorying */
> sbi->s_journal_destorying = true;
> jbd2_journal_destroy()
> journal->j_flags |= JBD2_UNMOUNT;
>
> jbd2_journal_start()
> start_this_handle()
> BUG_ON(JBD2_UNMOUNT)
>
> Thanks,
> Yi.
Hi Yi,
Yes you are right, somehow missed this edge case :(
Alright then, we have to move out sbi->s_journal_destroying outside the
helper. Just wondering if I should still let it be in
ext4_journal_destroy and just add an extra s_journal_destroying = false
before schedule_work(s_sb_upd_work), because it makes sense.
Okay let me give it some thought but thanks for pointing this out!
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;
>
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