[PATCH AUTOSEL 6.18-5.10] block,bfq: fix aux stat accumulation destination

[Sasha Levin <sashal@kernel.org>]

From: shechenglong <shechenglong@xfusion.com>

[ Upstream commit 04bdb1a04d8a2a89df504c1e34250cd3c6e31a1c ]

Route bfqg_stats_add_aux() time accumulation into the destination
stats object instead of the source, aligning with other stat fields.

Reviewed-by: Yu Kuai <yukuai@fnnas.com>
Signed-off-by: shechenglong <shechenglong@xfusion.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---

LLM Generated explanations, may be completely bogus:

## Analysis of Commit: block,bfq: fix aux stat accumulation destination

### 1. COMMIT MESSAGE ANALYSIS

The commit message clearly indicates this is a bug fix: "fix aux stat
accumulation destination". It describes that `bfqg_stats_add_aux()` was
routing time accumulation into the wrong destination (source instead of
destination). The fix aligns this field with all other stat fields in
the same function.

- Has `Reviewed-by: Yu Kuai` - BFQ subsystem expertise
- Signed off by Jens Axboe (block subsystem maintainer)

### 2. CODE CHANGE ANALYSIS

The change is a single-line fix in `block/bfq-cgroup.c`:

```c
- bfq_stat_add_aux(&from->time, &from->time);
+       bfq_stat_add_aux(&to->time, &from->time);
```

**The Bug:** The function `bfqg_stats_add_aux()` is documented with `/*
@to += @from */` - it should add stats FROM source TO destination. The
buggy line was adding `from->time` to itself (`&from->time,
&from->time`), which is clearly wrong.

**Root Cause:** A simple typo - `from` was used instead of `to` for the
first argument.

**Pattern Evidence:** Looking at surrounding code, every other line
follows the correct pattern:
- `blkg_rwstat_add_aux(&to->merged, &from->merged)`
- `blkg_rwstat_add_aux(&to->service_time, &from->service_time)`
- `bfq_stat_add_aux(&to->avg_queue_size_sum, &from->avg_queue_size_sum)`
- etc.

Only `time` had the incorrect `&from` as the first argument.

### 3. CLASSIFICATION

- **Bug type:** Logic error (typo) causing incorrect stat accumulation
- **Category:** Clear bug fix - not a feature, not a cleanup
- **Subsystem:** BFQ I/O scheduler cgroup support (debug statistics)

### 4. SCOPE AND RISK ASSESSMENT

- **Lines changed:** 1 line, essentially a single character change
  (`from` → `to`)
- **Files touched:** 1 file
- **Risk level:** Extremely LOW
- **Code location:** Under `CONFIG_BFQ_CGROUP_DEBUG` - debug statistics
  only

This is about as surgical as a fix can get. Even if the fix were somehow
wrong (which it clearly isn't), it only affects debug statistics output,
not actual I/O scheduling behavior.

### 5. USER IMPACT

- **Affected users:** Those using BFQ I/O scheduler with cgroup
  debugging enabled
- **Symptom:** The `time` statistic would be lost (not transferred to
  parent) when a BFQ cgroup is destroyed, causing inaccurate cumulative
  statistics
- **Severity:** Low to moderate - affects accuracy of debug/monitoring
  data, not data integrity or system stability

### 6. STABILITY INDICATORS

- Reviewed by Yu Kuai (BFQ expert)
- Accepted by Jens Axboe (block maintainer)
- Trivially correct fix - the pattern is obvious from surrounding code

### 7. DEPENDENCY CHECK

- **Dependencies:** None - this is a self-contained fix
- **Affected code existence:** BFQ cgroup code has existed in stable
  trees for years
- **Clean backport:** Should apply cleanly to any kernel with BFQ cgroup
  support

### VERDICT

**Pros:**
- Obviously correct fix (typo/copy-paste error)
- Minimal change (1 line)
- Zero regression risk
- Fixes incorrect behavior in statistics accumulation
- Well-reviewed and accepted by maintainers
- No dependencies on other commits

**Cons:**
- Only affects debug code (`CONFIG_BFQ_CGROUP_DEBUG`)
- Low-impact bug (statistics accuracy, not data/system integrity)

This commit meets all stable kernel criteria:
1. ✅ Obviously correct - trivially evident typo fix
2. ✅ Fixes a real bug - stats were accumulated to wrong destination
3. ✅ Small and contained - single line change
4. ✅ No new features - pure bug fix
5. ✅ Should apply cleanly to stable

While the impact is limited to debug statistics users, the fix is so
trivially correct and low-risk that there's no reason not to backport
it. Users relying on BFQ cgroup statistics for monitoring or debugging
would benefit from accurate data.

**YES**

 block/bfq-cgroup.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/block/bfq-cgroup.c b/block/bfq-cgroup.c
index 9fb9f3533150..6a75fe1c7a5c 100644
--- a/block/bfq-cgroup.c
+++ b/block/bfq-cgroup.c
@@ -380,7 +380,7 @@ static void bfqg_stats_add_aux(struct bfqg_stats *to, struct bfqg_stats *from)
 	blkg_rwstat_add_aux(&to->merged, &from->merged);
 	blkg_rwstat_add_aux(&to->service_time, &from->service_time);
 	blkg_rwstat_add_aux(&to->wait_time, &from->wait_time);
-	bfq_stat_add_aux(&from->time, &from->time);
+	bfq_stat_add_aux(&to->time, &from->time);
 	bfq_stat_add_aux(&to->avg_queue_size_sum, &from->avg_queue_size_sum);
 	bfq_stat_add_aux(&to->avg_queue_size_samples,
 			  &from->avg_queue_size_samples);
-- 
2.51.0