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Message-ID: <5b3864c3-bcfd-4f45-b427-224d32aca478@huaweicloud.com>
Date: Fri, 7 Mar 2025 10:49:28 +0800
From: Zhang Yi <yi.zhang@...weicloud.com>
To: Ojaswin Mujoo <ojaswin@...ux.ibm.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 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.
> 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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