linux-ext4 - Re: [PATCH 02/12] ext4: Remove redundant sb checksum recomputation

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Message-Id: <301139FC-B346-4199-B26E-1FF0CB970746@dilger.ca>
Date:   Sun, 29 Nov 2020 15:11:05 -0700
From:   Andreas Dilger <adilger@...ger.ca>
To:     Jan Kara <jack@...e.cz>, Eric Biggers <ebiggers@...nel.org>
Cc:     Ted Tso <tytso@....edu>, linux-ext4@...r.kernel.org
Subject: Re: [PATCH 02/12] ext4: Remove redundant sb checksum recomputation

On Nov 27, 2020, at 4:33 AM, Jan Kara <jack@...e.cz> wrote:
> 
> Superblock is written out either through ext4_commit_super() or through
> ext4_handle_dirty_super(). In both cases we recompute the checksum so it
> is not necessary to recompute it after updating superblock free inodes &
> blocks counters.

I searched through the code to see where s_sbh is being used, and it
looks like there is one case that doesn't update the checksum using
ext4_handle_dirty_super(), namely:

ext4_file_ioctl(cmd=FS_IOC_GET_ENCRYPTION_PWSALT)
{
                        err = ext4_journal_get_write_access(handle, sbi->s_sbh);
                        if (err)
                                goto pwsalt_err_journal;
                        generate_random_uuid(sbi->s_es->s_encrypt_pw_salt);
                        err = ext4_handle_dirty_metadata(handle, NULL,
                                                         sbi->s_sbh);

I don't think that is a problem with this patch, per se, but looks like
a bug that could be hit in rare cases with fscrypt + metadata_csum.  It
would only happen once per filesystem, and would normally be hidden by
later superblock updates, but should probably be fixed anyway.

Reviewed-by: Andreas Dilger <adilger@...ger.ca>

> Signed-off-by: Jan Kara <jack@...e.cz>
> ---
> fs/ext4/super.c | 2 --
> 1 file changed, 2 deletions(-)
> 
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index 2b08b162075c..61e6e5f156f3 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -5004,13 +5004,11 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
> 	block = ext4_count_free_clusters(sb);
> 	ext4_free_blocks_count_set(sbi->s_es,
> 				   EXT4_C2B(sbi, block));
> -	ext4_superblock_csum_set(sb);
> 	err = percpu_counter_init(&sbi->s_freeclusters_counter, block,
> 				  GFP_KERNEL);
> 	if (!err) {
> 		unsigned long freei = ext4_count_free_inodes(sb);
> 		sbi->s_es->s_free_inodes_count = cpu_to_le32(freei);
> -		ext4_superblock_csum_set(sb);
> 		err = percpu_counter_init(&sbi->s_freeinodes_counter, freei,
> 					  GFP_KERNEL);
> 	}
> --
> 2.16.4
> 


Cheers, Andreas






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