[PATCH 1/5] ext4: keep "prefetch_grp" and "nr" consistent

[Kemeng Shi]

Keep "prefetch_grp" and "nr" consistent to avoid to call
ext4_mb_prefetch_fini with non-prefetched groups.
When we step into next criteria, "prefetch_grp" is set to prefetch start
of new criteria while "nr" is number of the prefetched group in previous
criteria. If previous criteria and next criteria are both inexpensive
(< CR_GOAL_LEN_SLOW) and prefetch_ios reachs sbi->s_mb_prefetch_limit
in previous criteria, "prefetch_grp" and "nr" will be inconsistent and
may introduce unexpected cost to do ext4_mb_init_group for non-prefetched
groups.
Reset "nr" to 0 when we reset "prefetch_grp" to start of prefech in next
criteria to ensure "prefetch_grp" and "nr" are consistent.

Signed-off-by: Kemeng Shi <shikemeng@xxxxxxxxxxxxxxx>
---
 fs/ext4/mballoc.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 12b3f196010b..a61fc52956b2 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -2856,6 +2856,7 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
 		group = ac->ac_g_ex.fe_group;
 		ac->ac_groups_linear_remaining = sbi->s_mb_max_linear_groups;
 		prefetch_grp = group;
+		nr = 0;
 
 		for (i = 0, new_cr = cr; i < ngroups; i++,
 		     ext4_mb_choose_next_group(ac, &new_cr, &group, ngroups)) {
-- 
2.30.0