linux-kernel - Re: [PATCH v6 05/11] ext4: Separate block bitmap and buddy bitmap freeing in ext4_mb_clear

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Message-Id: <878r9q8cf9.fsf@doe.com>
Date:   Fri, 01 Sep 2023 15:04:18 +0530
From:   Ritesh Harjani (IBM) <ritesh.list@...il.com>
To:     Kemeng Shi <shikemeng@...weicloud.com>, tytso@....edu,
        adilger.kernel@...ger.ca, linux-ext4@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 05/11] ext4: Separate block bitmap and buddy bitmap freeing in ext4_mb_clear_bb()

Kemeng Shi <shikemeng@...weicloud.com> writes:

> This patch separates block bitmap and buddy bitmap freeing in order to
> udpate block bitmap with ext4_mb_mark_context in following patch.
^^^ update.

>
> Separated freeing is safe with concurrent allocation as long as:
> 1. Firstly allocate block in buddy bitmap, and then in block bitmap.
> 2. Firstly free block in block bitmap, and then buddy bitmap.
> Then freed block will only be available to allocation when both buddy
> bitmap and block bitmap are updated by freeing.
> Allocation obeys rule 1 already, just do sperated freeing with rule 2.

So we also don't need ext4_load_buddy_gfp() before freeing on-disk
bitmap right. Continue below...

>
> Separated freeing has no race with generate_buddy as:
> Once ext4_mb_load_buddy_gfp is executed successfully, the update-to-date
> buddy page can be found in sbi->s_buddy_cache and no more buddy
> initialization of the buddy page will be executed concurrently until
> buddy page is unloaded. As we always do free in "load buddy, free,
> unload buddy" sequence, separated freeing has no race with generate_buddy.
>
> Signed-off-by: Kemeng Shi <shikemeng@...weicloud.com>
> ---
>  fs/ext4/mballoc.c | 18 ++++++++----------
>  1 file changed, 8 insertions(+), 10 deletions(-)
>
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index e650eac22237..01ad36a1cc96 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -6519,6 +6519,14 @@ static void ext4_mb_clear_bb(handle_t *handle, struct inode *inode,
>  	if (err)
>  		goto error_return;


Let me add the a piece of code before the added changes and continue...

	err = ext4_mb_load_buddy_gfp(sb, block_group, &e4b,
				     GFP_NOFS|__GFP_NOFAIL);
	if (err)
		goto error_return;
>  
> +	ext4_lock_group(sb, block_group);
> +	mb_clear_bits(bitmap_bh->b_data, bit, count_clusters);
> +	ret = ext4_free_group_clusters(sb, gdp) + count_clusters;
> +	ext4_free_group_clusters_set(sb, gdp, ret);
> +	ext4_block_bitmap_csum_set(sb, gdp, bitmap_bh);
> +	ext4_group_desc_csum_set(sb, block_group, gdp);
> +	ext4_unlock_group(sb, block_group);
> +

...Is it required for ext4_mb_load_buddy_gfp() to be called before
freeing on-disk bitmap blocks? Can you explain why please?
At least it is not very clear to me that why do we need
ext4_mb_load_buddy_gfp() before clearing of bitmap blocks. If it's not
needed then I think we should separate out bitmap freeing logic and
buddy freeing logic even further.

Thoughts?

>  	/*
>  	 * We need to make sure we don't reuse the freed block until after the
>  	 * transaction is committed. We make an exception if the inode is to be
> @@ -6541,13 +6549,8 @@ static void ext4_mb_clear_bb(handle_t *handle, struct inode *inode,
>  		new_entry->efd_tid = handle->h_transaction->t_tid;
>  
>  		ext4_lock_group(sb, block_group);
> -		mb_clear_bits(bitmap_bh->b_data, bit, count_clusters);
>  		ext4_mb_free_metadata(handle, &e4b, new_entry);
>  	} else {
> -		/* need to update group_info->bb_free and bitmap
> -		 * with group lock held. generate_buddy look at
> -		 * them with group lock_held
> -		 */
>  		if (test_opt(sb, DISCARD)) {
>  			err = ext4_issue_discard(sb, block_group, bit,
>  						 count_clusters, NULL);
> @@ -6560,14 +6563,9 @@ static void ext4_mb_clear_bb(handle_t *handle, struct inode *inode,
>  			EXT4_MB_GRP_CLEAR_TRIMMED(e4b.bd_info);
>  
>  		ext4_lock_group(sb, block_group);
> -		mb_clear_bits(bitmap_bh->b_data, bit, count_clusters);
>  		mb_free_blocks(inode, &e4b, bit, count_clusters);
>  	}
>  
> -	ret = ext4_free_group_clusters(sb, gdp) + count_clusters;
> -	ext4_free_group_clusters_set(sb, gdp, ret);
> -	ext4_block_bitmap_csum_set(sb, gdp, bitmap_bh);
> -	ext4_group_desc_csum_set(sb, block_group, gdp);
>  	ext4_unlock_group(sb, block_group);
>  
>  	if (sbi->s_log_groups_per_flex) {


Adding piece of code here...

	if (sbi->s_log_groups_per_flex) {
		ext4_group_t flex_group = ext4_flex_group(sbi, block_group);
		atomic64_add(count_clusters,
			     &sbi_array_rcu_deref(sbi, s_flex_groups,
						  flex_group)->free_clusters);
	}

<...>

	/* We dirtied the bitmap block */
	BUFFER_TRACE(bitmap_bh, "dirtied bitmap block");
	err = ext4_handle_dirty_metadata(handle, NULL, bitmap_bh);
	/* And the group descriptor block */
	BUFFER_TRACE(gd_bh, "dirtied group descriptor block");
	ret = ext4_handle_dirty_metadata(handle, NULL, gd_bh);
	if (!err)
		err = ret;

I was thinking even this can go around bitmap freeing logic above. So
the next patch becomes very clear that all of the bitmap freeing logic
is just simply moved into ext4_mb_mark_context() function.

-ritesh