linux-kernel - [PATCH] btrfs: reset block group size class when reservations are freed

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Message-Id: <20260110203340.12443-1-jiashengjiangcool@gmail.com>
Date: Sat, 10 Jan 2026 20:33:40 +0000
From: Jiasheng Jiang <jiashengjiangcool@...il.com>
To: Chris Mason <clm@...com>,
	David Sterba <dsterba@...e.com>
Cc: linux-btrfs@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Jiasheng Jiang <jiashengjiangcool@...il.com>
Subject: [PATCH] btrfs: reset block group size class when reservations are freed

Differential analysis of block-group.c shows an inconsistency between
btrfs_add_reserved_bytes() and btrfs_free_reserved_bytes().

When space is reserved, btrfs_use_block_group_size_class() is called to
set a block group's size class, specializing it for a specific allocation
size to reduce fragmentation. However, when these reservations are
subsequently freed (e.g., due to an error or transaction abort),
btrfs_free_reserved_bytes() fails to perform the corresponding cleanup.

This leads to a state leak where a block group remains stuck with a
specific size class even if it contains no used or reserved bytes. This
stale state causes find_free_extent to unnecessarily skip these block
groups for mismatched size requests, leading to suboptimal allocation
behavior.

Fix this by resetting the size class to BTRFS_BG_SZ_NONE in
btrfs_free_reserved_bytes() when the block group becomes completely
empty.

Signed-off-by: Jiasheng Jiang <jiashengjiangcool@...il.com>
---
 fs/btrfs/block-group.c | 11 +++++++++++
 fs/btrfs/block-group.h |  1 +
 2 files changed, 12 insertions(+)

diff --git a/fs/btrfs/block-group.c b/fs/btrfs/block-group.c
index 08b14449fabe..1ecac4613a3e 100644
--- a/fs/btrfs/block-group.c
+++ b/fs/btrfs/block-group.c
@@ -3865,6 +3865,10 @@ void btrfs_free_reserved_bytes(struct btrfs_block_group *cache, u64 num_bytes,

 	spin_lock(&space_info->lock);
 	spin_lock(&cache->lock);
+
+	if (btrfs_block_group_should_use_size_class(cache))
+		btrfs_maybe_reset_size_class(cache);
+
 	bg_ro = cache->ro;
 	cache->reserved -= num_bytes;
 	if (is_delalloc)
@@ -4717,3 +4721,10 @@ bool btrfs_block_group_should_use_size_class(const struct btrfs_block_group *bg)
 		return false;
 	return true;
 }
+
+void btrfs_maybe_reset_size_class(struct btrfs_block_group *bg)
+{
+	lockdep_assert_held(&bg->lock);
+	if (bg->used == 0 && bg->reserved == 0)
+		bg->size_class = BTRFS_BG_SZ_NONE;
+}
diff --git a/fs/btrfs/block-group.h b/fs/btrfs/block-group.h
index 5f933455118c..7e02db8a8bc6 100644
--- a/fs/btrfs/block-group.h
+++ b/fs/btrfs/block-group.h
@@ -395,5 +395,6 @@ int btrfs_use_block_group_size_class(struct btrfs_block_group *bg,
 				     enum btrfs_block_group_size_class size_class,
 				     bool force_wrong_size_class);
 bool btrfs_block_group_should_use_size_class(const struct btrfs_block_group *bg);
+void btrfs_maybe_reset_size_class(struct btrfs_block_group *bg);

 #endif /* BTRFS_BLOCK_GROUP_H */
-- 
2.25.1