linux-ext4 - Re: [PATCH v2 02/12] jbd2: add fast commit fields to journal

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Date:   Fri, 9 Aug 2019 13:48:43 -0600
From:   Andreas Dilger <adilger@...ger.ca>
To:     Harshad Shirwadkar <harshadshirwadkar@...il.com>
Cc:     linux-ext4@...r.kernel.org
Subject: Re: [PATCH v2 02/12] jbd2: add fast commit fields to journal_s
 structure

On Aug 8, 2019, at 9:45 PM, Harshad Shirwadkar <harshadshirwadkar@...il.com> wrote:
> 
> For fast commits, JBD2 as of now allocates a default of 128 blocks at
> the end of the journalling area. Although JBD2 owns these blocks, it
> doesn't control what exactly should be written in these blocks. It
> just provides the right abstraction for making these blocks usable by
> file systems. This patch adds necessary fields to manage these fast
> commit blocks.
> 
> Signed-off-by: Harshad Shirwadkar <harshadshirwadkar@...il.com>

Contrary to the description, this patch doesn't really do anything beyond
adding unused fields and constants to the header, and as such isn't really
useful to land on its own since there is no way to see what the fields
are used for.  In particular, I was going to say that JBD2_FAST_COMMIT_BLOCKS
should only be reserved if the FAST_COMMIT feature is enabled (unlike what
is written above, which implies that they are always reserved), otherwise
it can impact filesystem performance even when the feature is not active.

I'd recommend to merge these changes with the patch where the fields/constants
are actually used.

Cheers, Andreas

> ---
> 
> Changelog:
> 
> V2: Added struct transaction_run_stats_s * argument to
>    j_fc_commit_callback to collect stats
> ---
> include/linux/jbd2.h | 79 ++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 79 insertions(+)
> 
> diff --git a/include/linux/jbd2.h b/include/linux/jbd2.h
> index b7eed49b8ecd..9a750b732241 100644
> --- a/include/linux/jbd2.h
> +++ b/include/linux/jbd2.h
> @@ -66,6 +66,7 @@ void __jbd2_debug(int level, const char *file, const char *func,
> extern void *jbd2_alloc(size_t size, gfp_t flags);
> extern void jbd2_free(void *ptr, size_t size);
> 
> +#define JBD2_FAST_COMMIT_BLOCKS 128
> #define JBD2_MIN_JOURNAL_BLOCKS 1024
> 
> #ifdef __KERNEL__
> @@ -918,6 +919,34 @@ struct journal_s
> 	 */
> 	unsigned long		j_last;
> 
> +	/**
> +	 * @j_first_fc:
> +	 *
> +	 * The block number of the first fast commit block in the journal
> +	 */
> +	unsigned long		j_first_fc;
> +
> +	/**
> +	 * @j_current_fc:
> +	 *
> +	 * Journal fc block iterator
> +	 */
> +	unsigned long		j_fc_off;
> +
> +	/**
> +	 * @j_last_fc:
> +	 *
> +	 * The block number of the last fast commit block in the journal
> +	 */
> +	unsigned long		j_last_fc;
> +
> +	/**
> +	 * @j_do_full_commit:
> +	 *
> +	 * Force a full commit. If this flag is set JBD2 won't try fast commits
> +	 */
> +	bool			j_do_full_commit;
> +
> 	/**
> 	 * @j_dev: Device where we store the journal.
> 	 */
> @@ -987,6 +1016,15 @@ struct journal_s
> 	 */
> 	tid_t			j_transaction_sequence;
> 
> +	/**
> +	 * @j_subtid:
> +	 *
> +	 * One plus the sequence number of the most recently committed fast
> +	 * commit. This represents the sub transaction ID for the next fast
> +	 * commit.
> +	 */
> +	tid_t			j_subtid;
> +
> 	/**
> 	 * @j_commit_sequence:
> 	 *
> @@ -1068,6 +1106,20 @@ struct journal_s
> 	 */
> 	int			j_wbufsize;
> 
> +	/**
> +	 * @j_fc_wbuf:
> +	 *
> +	 * Array of bhs for fast commit transactions
> +	 */
> +	struct buffer_head	**j_fc_wbuf;
> +
> +	/**
> +	 * @j_fc_wbufsize:
> +	 *
> +	 * Size of @j_fc_wbufsize array.
> +	 */
> +	int			j_fc_wbufsize;
> +
> 	/**
> 	 * @j_last_sync_writer:
> 	 *
> @@ -1167,6 +1219,33 @@ struct journal_s
> 	 */
> 	struct lockdep_map	j_trans_commit_map;
> #endif
> +	/**
> +	 * @j_fc_commit_callback:
> +	 *
> +	 * File-system specific function that performs actual fast commit
> +	 * operation. Should return 0 if the fast commit was successful, in that
> +	 * case, JBD2 will just increment journal->j_subtid and move on. If it
> +	 * returns < 0, JBD2 will fall-back to full commit.
> +	 */
> +	int (*j_fc_commit_callback)(struct journal_s *journal, tid_t tid,
> +				    tid_t subtid,
> +				    struct transaction_run_stats_s *stats);
> +	/**
> +	 * @j_fc_replay_callback:
> +	 *
> +	 * File-system specific function that performs replay of a fast
> +	 * commit. JBD2 calls this function for each fast commit block found in
> +	 * the journal.
> +	 */
> +	int (*j_fc_replay_callback)(struct journal_s *journal,
> +				    struct buffer_head *bh);
> +	/**
> +	 * @j_fc_cleanup_callback:
> +	 *
> +	 * Clean-up after fast commit or full commit. JBD2 calls this function
> +	 * after every commit operation.
> +	 */
> +	void (*j_fc_cleanup_callback)(struct journal_s *journal);
> };
> 
> #define jbd2_might_wait_for_commit(j) \
> --
> 2.23.0.rc1.153.gdeed80330f-goog
> 


Cheers, Andreas






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