linux-ext4 - Re: [PATCH] jbd2: fix the inconsistency between checksum and data in memory for journal sb

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Message-ID: <cf167bfa-fb1e-40fa-81e6-071dab2442c0@huaweicloud.com>
Date: Wed, 29 Oct 2025 16:45:34 +0800
From: Zhang Yi <yi.zhang@...weicloud.com>
To: Ye Bin <yebin@...weicloud.com>, linux-ext4@...r.kernel.org
Cc: jack@...e.cz, tytso@....edu, adilger.kernel@...ger.ca
Subject: Re: [PATCH] jbd2: fix the inconsistency between checksum and data in
 memory for journal sb

Hi Bin!

On 10/28/2025 2:47 PM, Ye Bin wrote:
> From: Ye Bin <yebin10@...wei.com>
> 
> Copying the file system while it is mounted as read-only results in
> a mount failure:
> [~]# mkfs.ext4 -F /dev/sdc
> [~]# mount /dev/sdc -o ro /mnt/test
> [~]# dd if=/dev/sdc of=/dev/sda bs=1M
> [~]# mount /dev/sda /mnt/test1
> [ 1094.849826] JBD2: journal checksum error
> [ 1094.850927] EXT4-fs (sda): Could not load journal inode
> mount: mount /dev/sda on /mnt/test1 failed: Bad message
> 

I suppose we need to explain why we have this particular use case.

> Above issue may happen as follows:
> ext4_fill_super
>  set_journal_csum_feature_set(sb)
>   if (ext4_has_metadata_csum(sb))
>    incompat = JBD2_FEATURE_INCOMPAT_CSUM_V3;
>   if (test_opt(sb, JOURNAL_CHECKSUM)
>    jbd2_journal_set_features(sbi->s_journal, compat, 0, incompat);
>     lock_buffer(journal->j_sb_buffer);
>     sb->s_feature_incompat  |= cpu_to_be32(incompat);
>     //The data in the journal sb was modified, but the checksum was not
>       updated, so the data remaining in memory has a mismatch between the
>       data and the checksum.
>     unlock_buffer(journal->j_sb_buffer);
> 
> In this case, the journal sb copied over is in a state where the checksum
> and data are inconsistent, so mounting fails.
> To solve the above issue, update the checksum in memory after modifying
> the journal sb.

What about checking sb_rdonly(sb) before setting the journal feature, and
skipping the setting if the filesystem is mounted as read-only?

Regards,
Yi.

> 
> Fixes: 4fd5ea43bc11 ("jbd2: checksum journal superblock")
> Signed-off-by: Ye Bin <yebin10@...wei.com>
> ---
>  fs/jbd2/journal.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
> index d480b94117cd..5b6e8c1a5e6a 100644
> --- a/fs/jbd2/journal.c
> +++ b/fs/jbd2/journal.c
> @@ -2349,6 +2349,8 @@ int jbd2_journal_set_features(journal_t *journal, unsigned long compat,
>  	sb->s_feature_compat    |= cpu_to_be32(compat);
>  	sb->s_feature_ro_compat |= cpu_to_be32(ro);
>  	sb->s_feature_incompat  |= cpu_to_be32(incompat);
> +	if (jbd2_journal_has_csum_v2or3(journal))
> +		sb->s_checksum = jbd2_superblock_csum(sb);
>  	unlock_buffer(journal->j_sb_buffer);
>  	jbd2_journal_init_transaction_limits(journal);
>  
> @@ -2378,9 +2380,13 @@ void jbd2_journal_clear_features(journal_t *journal, unsigned long compat,
>  
>  	sb = journal->j_superblock;
>  
> +	lock_buffer(journal->j_sb_buffer);
>  	sb->s_feature_compat    &= ~cpu_to_be32(compat);
>  	sb->s_feature_ro_compat &= ~cpu_to_be32(ro);
>  	sb->s_feature_incompat  &= ~cpu_to_be32(incompat);
> +	if (jbd2_journal_has_csum_v2or3(journal))
> +		sb->s_checksum = jbd2_superblock_csum(sb);
> +	unlock_buffer(journal->j_sb_buffer);
>  	jbd2_journal_init_transaction_limits(journal);
>  }
>  EXPORT_SYMBOL(jbd2_journal_clear_features);