[PATCH v2 03/16] ext4: remove unnecessary s_md_lock on update s_mb_last_group

Baokun Li
2025-06-23T07:47:00Z

After we optimized the block group lock, we found another lock
contention issue when running will-it-scale/fallocate2 with multiple
processes. The fallocate's block allocation and the truncate's block
release were fighting over the s_md_lock. The problem is, this lock
protects totally different things in those two processes: the list of
freed data blocks (s_freed_data_list) when releasing, and where to start
looking for new blocks (mb_last_group) when allocating.

Now we only need to track s_mb_last_group and no longer need to track
s_mb_last_start, so we don't need the s_md_lock lock to ensure that the
two are consistent, and we can ensure that the s_mb_last_group read is up
to date by using smp_store_release/smp_load_acquire.

Besides, the s_mb_last_group data type only requires ext4_group_t
(i.e., unsigned int), rendering unsigned long superfluous.

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 | 4821 | 7612 (+57.8%) | 15371 | 21647 (+40.8%) |
mb_optimize_scan=1 | 4784 | 7568 (+58.1%) | 6101 | 9117 (+49.4%) |

Signed-off-by: Baokun Li <libaokun1@huawei.com>
---
fs/ext4/ext4.h | 2 +-
fs/ext4/mballoc.c | 17 ++++++-----------
2 files changed, 7 insertions(+), 12 deletions(-)

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index cfb60f8fbb63..93f03d8c3dca 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -1630,7 +1630,7 @@ struct ext4_sb_info {
unsigned int s_mb_group_prealloc;
unsigned int s_max_dir_size_kb;
/* where last allocation was done - for stream allocation */
- unsigned long s_mb_last_group;
+ ext4_group_t s_mb_last_group;
unsigned int s_mb_prefetch;
unsigned int s_mb_prefetch_limit;
unsigned int s_mb_best_avail_max_trim_order;
diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 5cdae3bda072..3f103919868b 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -2168,11 +2168,9 @@ static void ext4_mb_use_best_found(struct ext4_allocation_context *ac,
ac->ac_buddy_folio = e4b->bd_buddy_folio;
folio_get(ac->ac_buddy_folio);
/* store last allocated for subsequent stream allocation */
- if (ac->ac_flags & EXT4_MB_STREAM_ALLOC) {
- spin_lock(&sbi->s_md_lock);
- sbi->s_mb_last_group = ac->ac_f_ex.fe_group;
- spin_unlock(&sbi->s_md_lock);
- }
+ if (ac->ac_flags & EXT4_MB_STREAM_ALLOC)
+ /* pairs with smp_load_acquire in ext4_mb_regular_allocator() */
+ smp_store_release(&sbi->s_mb_last_group, ac->ac_f_ex.fe_group);
/*
* As we've just preallocated more space than
* user requested originally, we store allocated
@@ -2844,12 +2842,9 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
}

/* if stream allocation is enabled, use global goal */
- if (ac->ac_flags & EXT4_MB_STREAM_ALLOC) {
- /* TBD: may be hot point */
- spin_lock(&sbi->s_md_lock);
- ac->ac_g_ex.fe_group = sbi->s_mb_last_group;
- spin_unlock(&sbi->s_md_lock);
- }
+ if (ac->ac_flags & EXT4_MB_STREAM_ALLOC)
+ /* pairs with smp_store_release in ext4_mb_use_best_found() */
+ ac->ac_g_ex.fe_group = smp_load_acquire(&sbi->s_mb_last_group);

/*
* Let's just scan groups to find more-less suitable blocks We
--
2.46.1