linux-kernel - [PATCH v6 07/11] ext4: Separate block bitmap and buddy bitmap freeing in ext4_group_add

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Message-Id: <20230826155028.4019470-8-shikemeng@huaweicloud.com>
Date:   Sat, 26 Aug 2023 23:50:24 +0800
From:   Kemeng Shi <shikemeng@...weicloud.com>
To:     tytso@....edu, adilger.kernel@...ger.ca, ritesh.list@...il.com,
        linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH v6 07/11] ext4: Separate block bitmap and buddy bitmap freeing in ext4_group_add_blocks()

This patch separates block bitmap and buddy bitmap freeing in order to
udpate block bitmap with ext4_mb_mark_context in following patch.
The reason why this can be sperated is explained in previous submit.
Put the explanation here to simplify the code archeology to
ext4_group_add_blocks():

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.

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 | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 25265531cb6a..bb08a71a6e61 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -6764,20 +6764,19 @@ int ext4_group_add_blocks(handle_t *handle, struct super_block *sb,
 	if (err)
 		goto error_return;

-	/*
-	 * need to update group_info->bb_free and bitmap
-	 * with group lock held. generate_buddy look at
-	 * them with group lock_held
-	 */
 	ext4_lock_group(sb, block_group);
 	mb_clear_bits(bitmap_bh->b_data, bit, cluster_count);
-	mb_free_blocks(NULL, &e4b, bit, cluster_count);
 	free_clusters_count = clusters_freed +
 		ext4_free_group_clusters(sb, desc);
 	ext4_free_group_clusters_set(sb, desc, free_clusters_count);
 	ext4_block_bitmap_csum_set(sb, desc, bitmap_bh);
 	ext4_group_desc_csum_set(sb, block_group, desc);
 	ext4_unlock_group(sb, block_group);
+
+	ext4_lock_group(sb, block_group);
+	mb_free_blocks(NULL, &e4b, bit, cluster_count);
+	ext4_unlock_group(sb, block_group);
+
 	percpu_counter_add(&sbi->s_freeclusters_counter,
 			   clusters_freed);

-- 
2.30.0