[PATCH v2 07/16] ext4: convert sbi->s_mb_free_pending to atomic_t

Baokun Li
2025-06-23T07:47:02Z

Previously, s_md_lock was used to protect s_mb_free_pending during
modifications, while smp_mb() ensured fresh reads, so s_md_lock just
guarantees the atomicity of s_mb_free_pending. Thus we optimized it by
converting s_mb_free_pending into an atomic variable, thereby eliminating
s_md_lock and minimizing lock contention. This also prepares for future
lockless merging of free extents.

Following this modification, s_md_lock is exclusively responsible for
managing insertions and deletions within s_freed_data_list, along with
operations involving list_splice.

Performance test data follows:

Test: Running will-it-scale/fallocate2 on CPU-bound containers.
Observation: Average fallocate operations per container per second.

| Kunpeng 920 / 512GB -P80| AMD 9654 / 1536GB -P96 |
Disk: 960GB SSD |-------------------------|-------------------------|
| base | patched | base | patched |
-------------------|-------|-----------------|-------|-----------------|
mb_optimize_scan=0 | 19699 | 20982 (+6.5%) | 53093 | 50629 (-4.6%) |
mb_optimize_scan=1 | 9862 | 10703 (+8.5%) | 14401 | 14856 (+3.1%) |

Signed-off-by: Baokun Li <libaokun1@huawei.com>
---
fs/ext4/balloc.c | 2 +-
fs/ext4/ext4.h | 2 +-
fs/ext4/mballoc.c | 9 +++------
3 files changed, 5 insertions(+), 8 deletions(-)

diff --git a/fs/ext4/balloc.c b/fs/ext4/balloc.c
index c48fd36b2d74..c9329ed5c094 100644
--- a/fs/ext4/balloc.c
+++ b/fs/ext4/balloc.c
@@ -703,7 +703,7 @@ int ext4_should_retry_alloc(struct super_block *sb, int *retries)
* possible we just missed a transaction commit that did so
*/
smp_mb();
- if (sbi->s_mb_free_pending == 0) {
+ if (atomic_read(&sbi->s_mb_free_pending) == 0) {
if (test_opt(sb, DISCARD)) {
atomic_inc(&sbi->s_retry_alloc_pending);
flush_work(&sbi->s_discard_work);
diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index 294198c05cdd..003b8d3726e8 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -1602,7 +1602,7 @@ struct ext4_sb_info {
unsigned short *s_mb_offsets;
unsigned int *s_mb_maxs;
unsigned int s_group_info_size;
- unsigned int s_mb_free_pending;
+ atomic_t s_mb_free_pending;
struct list_head s_freed_data_list[2]; /* List of blocks to be freed
after commit completed */
struct list_head s_discard_list;
diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 216b332a5054..5410fb3688ee 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -3680,7 +3680,7 @@ int ext4_mb_init(struct super_block *sb)
}

spin_lock_init(&sbi->s_md_lock);
- sbi->s_mb_free_pending = 0;
+ atomic_set(&sbi->s_mb_free_pending, 0);
INIT_LIST_HEAD(&sbi->s_freed_data_list[0]);
INIT_LIST_HEAD(&sbi->s_freed_data_list[1]);
INIT_LIST_HEAD(&sbi->s_discard_list);
@@ -3894,10 +3894,7 @@ static void ext4_free_data_in_buddy(struct super_block *sb,
/* we expect to find existing buddy because it's pinned */
BUG_ON(err != 0);

- spin_lock(&EXT4_SB(sb)->s_md_lock);
- EXT4_SB(sb)->s_mb_free_pending -= entry->efd_count;
- spin_unlock(&EXT4_SB(sb)->s_md_lock);
-
+ atomic_sub(entry->efd_count, &EXT4_SB(sb)->s_mb_free_pending);
db = e4b.bd_info;
/* there are blocks to put in buddy to make them really free */
count += entry->efd_count;
@@ -6392,7 +6389,7 @@ ext4_mb_free_metadata(handle_t *handle, struct ext4_buddy *e4b,

spin_lock(&sbi->s_md_lock);
list_add_tail(&new_entry->efd_list, &sbi->s_freed_data_list[new_entry->efd_tid & 1]);
- sbi->s_mb_free_pending += clusters;
+ atomic_add(clusters, &sbi->s_mb_free_pending);
spin_unlock(&sbi->s_md_lock);
}

--
2.46.1