linux-ext4 - Re: [PATCH] ext4: Add periodic superblock update check

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Date:   Wed, 2 Aug 2023 15:18:48 -0600
From:   Andreas Dilger <adilger@...ger.ca>
To:     Vitaliy Kuznetsov <vk.en.mail@...il.com>
Cc:     linux-ext4@...r.kernel.org
Subject: Re: [PATCH] ext4: Add periodic superblock update check

On Jul 31, 2023, at 6:25 AM, Vitaliy Kuznetsov <vk.en.mail@...il.com> wrote:
> 
> This patch introduces a mechanism to periodically check and update
> the superblock within the ext4 file system. The main purpose of this
> patch is to keep the disk superblock up to date. The update will be
> performed if more than one hour has passed since the last update, and
> if more than 16MB of data have been written to disk.
> 
> This check and update is performed within the ext4_journal_commit_callback
> function, ensuring that the superblock is written while the disk is
> active, rather than based on a timer that may trigger during disk idle
> periods.
> 
> Discussion https://www.spinics.net/lists/linux-ext4/msg85865.html
> 
> Signed-off-by: Vitaliy Kuznetsov <vk.en.mail@...il.com>

Thanks for sending the patch.

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

> ---
> fs/ext4/super.c | 52 +++++++++++++++++++++++++++++++++++++++++++++++++
> fs/ext4/sysfs.c |  4 ++--
> 2 files changed, 54 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index c94ebf704616..2159e9705404 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -433,6 +433,57 @@ static time64_t __ext4_get_tstamp(__le32 *lo, __u8 *hi)
> #define ext4_get_tstamp(es, tstamp) \
> 	__ext4_get_tstamp(&(es)->tstamp, &(es)->tstamp ## _hi)
> 
> +#define EXT4_SB_REFRESH_INTERVAL_SEC (3600) /* seconds (1 hour) */
> +#define EXT4_SB_REFRESH_INTERVAL_KB (16384) /* kilobytes (16MB) */
> +
> +/*
> + * The ext4_maybe_update_superblock() function checks and updates the
> + * superblock if needed.
> + *
> + * This function is designed to update the on-disk superblock only under
> + * certain conditions to prevent excessive disk writes and unnecessary
> + * waking of the disk from sleep. The superblock will be updated if:
> + * 1. More than an hour has passed since the last superblock update, and
> + * 2. More than 16MB have been written since the last superblock update.
> + *
> + * @sb: The superblock
> + */
> +static void ext4_maybe_update_superblock(struct super_block *sb)
> +{
> +	struct ext4_sb_info *sbi = EXT4_SB(sb);
> +	struct ext4_super_block *es = sbi->s_es;
> +	journal_t *journal = sbi->s_journal;
> +	time64_t now;
> +	__u64 last_update;
> +	__u64 lifetime_write_kbytes;
> +	__u64 diff_size;
> +
> +	if (sb_rdonly(sb) || !(sb->s_flags & SB_ACTIVE) ||
> +	    !journal || (journal->j_flags & JBD2_UNMOUNT))
> +		return;
> +
> +	now = ktime_get_real_seconds();
> +	last_update = ext4_get_tstamp(es, s_wtime);
> +
> +	if (likely(now - last_update < EXT4_SB_REFRESH_INTERVAL_SEC))
> +		return;
> +
> +	lifetime_write_kbytes = sbi->s_kbytes_written +
> +		((part_stat_read(sb->s_bdev, sectors[STAT_WRITE]) -
> +		  sbi->s_sectors_written_start) >> 1);
> +
> +	/* Get the number of kilobytes not written to disk to account
> +	 * for statistics and compare with a multiple of 16 MB. This
> +	 * is used to determine when the next superblock commit should
> +	 * occur (i.e. not more often than once per 16MB if there was
> +	 * less written in an hour).
> +	 */
> +	diff_size = lifetime_write_kbytes - le64_to_cpu(es->s_kbytes_written);
> +
> +	if (diff_size > EXT4_SB_REFRESH_INTERVAL_KB)
> +		schedule_work(&EXT4_SB(sb)->s_error_work);
> +}
> +
> /*
>  * The del_gendisk() function uninitializes the disk-specific data
>  * structures, including the bdi structure, without telling anyone
> @@ -459,6 +510,7 @@ static void ext4_journal_commit_callback(journal_t *journal, transaction_t *txn)
> 	BUG_ON(txn->t_state == T_FINISHED);
> 
> 	ext4_process_freed_data(sb, txn->t_tid);
> +	ext4_maybe_update_superblock(sb);
> 
> 	spin_lock(&sbi->s_md_lock);
> 	while (!list_empty(&txn->t_private_list)) {
> diff --git a/fs/ext4/sysfs.c b/fs/ext4/sysfs.c
> index 6d332dff79dd..9f334de4f636 100644
> --- a/fs/ext4/sysfs.c
> +++ b/fs/ext4/sysfs.c
> @@ -515,7 +515,8 @@ static const struct kobj_type ext4_feat_ktype = {
> 
> void ext4_notify_error_sysfs(struct ext4_sb_info *sbi)
> {
> -	sysfs_notify(&sbi->s_kobj, NULL, "errors_count");
> +	if (sbi->s_add_error_count > 0)
> +		sysfs_notify(&sbi->s_kobj, NULL, "errors_count");
> }
> 
> static struct kobject *ext4_root;
> @@ -605,4 +606,3 @@ void ext4_exit_sysfs(void)
> 	remove_proc_entry(proc_dirname, NULL);
> 	ext4_proc_root = NULL;
> }
> -
> --


Cheers, Andreas






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