ext4: remove unnecessary s_md_lock on update s_mb_last_group

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. Since s_mb_last_group is merely a hint and doesn't
require strong synchronization, READ_ONCE/WRITE_ONCE is sufficient.

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.

|CPU: Kunpeng 920   |          P80           |            P1           |
|Memory: 512GB      |------------------------|-------------------------|
|960GB SSD (0.5GB/s)| base  |    patched     | base   |    patched     |
|-------------------|-------|----------------|--------|----------------|
|mb_optimize_scan=0 | 4821  | 9636  (+99.8%) | 314065 | 337597 (+7.4%) |
|mb_optimize_scan=1 | 4784  | 4834  (+1.04%) | 316344 | 341440 (+7.9%) |

|CPU: AMD 9654 * 2  |          P96           |             P1          |
|Memory: 1536GB     |------------------------|-------------------------|
|960GB SSD (1GB/s)  | base  |    patched     | base   |    patched     |
|-------------------|-------|----------------|--------|----------------|
|mb_optimize_scan=0 | 15371 | 22341 (+45.3%) | 205851 | 219707 (+6.7%) |
|mb_optimize_scan=1 | 6101  | 9177  (+50.4%) | 207373 | 215732 (+4.0%) |

Suggested-by: Jan Kara <jack@suse.cz>
Signed-off-by: Baokun Li <libaokun1@huawei.com>
Reviewed-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Link: https://patch.msgid.link/20250714130327.1830534-5-libaokun1@huawei.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

Author: LiBaokun96

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;

fs/ext4/mballoc.c

@@ -2168,11 +2168,8 @@ 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)
+		WRITE_ONCE(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
@@ -2845,10 +2842,7 @@ 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);
+		ac->ac_g_ex.fe_group = READ_ONCE(sbi->s_mb_last_group);
 		ac->ac_g_ex.fe_start = -1;
 		ac->ac_flags &= ~EXT4_MB_HINT_TRY_GOAL;
 	}