From: Yu Kuai yukuai3@huawei.com
[ Upstream commit 2afe17794cfed5f80295b1b9facd66e6f65e5002 ]
It's supposed to be COUNTER_MAX / 2, not COUNTER_MAX.
Link: https://lore.kernel.org/linux-raid/20250524061320.370630-14-yukuai1@huaweicl... Signed-off-by: Yu Kuai yukuai3@huawei.com Reviewed-by: Christoph Hellwig hch@lst.de Reviewed-by: Hannes Reinecke hare@suse.de Signed-off-by: Sasha Levin sashal@kernel.org ---
Now I have a clear understanding. Let me analyze the specific code change in the commit:
**YES**
This commit should be backported to stable kernel trees. Here's my extensive analysis:
## Analysis of the Bug Fix
### 1. The Bug The code in `md_bitmap_new_disk_sb()` incorrectly validates `write_behind` against `COUNTER_MAX` instead of `COUNTER_MAX / 2`:
```c - if (write_behind > COUNTER_MAX) + if (write_behind > COUNTER_MAX / 2) write_behind = COUNTER_MAX / 2; ```
### 2. Why This is a Real Bug
The comment immediately above this code explicitly states: ```c /* - FIXME: write_behind for RAID1. If not specified, what - is a good choice? We choose COUNTER_MAX / 2 arbitrarily. */ ```
The intent is clearly to limit write_behind to `COUNTER_MAX / 2`, but the validation check was incorrectly comparing against `COUNTER_MAX`. This creates an inconsistency where: - Values between `COUNTER_MAX / 2` and `COUNTER_MAX` would pass the check - But then get clamped to `COUNTER_MAX / 2` anyway - This means the validation isn't serving its intended purpose
### 3. Impact on Users
This bug affects: - **dm-raid users**: When creating new bitmap superblocks for RAID arrays with write-behind configured - **Data consistency**: While not causing data corruption, it creates confusion about the actual limits - **Configuration validation**: Users might set values that appear to be accepted but are silently reduced
### 4. Consistency with Other Code
The fix aligns with the pattern used elsewhere: - In `dm-raid.c`: The validation correctly checks `value / 2 > COUNTER_MAX` (equivalent check for sector-based values) - The clamping value of `COUNTER_MAX / 2` is consistently used as the default throughout the codebase
### 5. Backport Criteria
This commit meets the stable kernel backport criteria: - **It fixes a bug**: Incorrect validation logic that doesn't match the intended behavior - **Simple and contained**: One-line change with minimal risk - **Clear fix**: The correct value is obvious from the surrounding code and comment - **User-visible impact**: Affects configuration validation for dm-raid users
### 6. Similar Historical Commits
Looking at the similar commits provided: - Commit #1 (backported): Added validation to prevent setting max_write_behind without write-mostly devices - Commit #4 (backported): Fixed missing bitmap updates with write-mostly devices
Both of these were backported and dealt with correctness issues in the write-behind functionality, establishing a pattern that such fixes are considered stable-worthy.
### 7. Risk Assessment
- **Low risk**: Single character change ('/' to '/ 2') - **Well-understood**: The correct limit is documented in the comment - **No ABI changes**: Just tightens an existing validation check - **Fail-safe**: Makes the validation more restrictive, not less
## Conclusion
This is a clear bug fix that corrects an incorrect validation check to match the documented and intended behavior. The fix is trivial, safe, and addresses a real issue that affects dm-raid users. It should be backported to ensure consistent behavior across stable kernel versions.
drivers/md/md-bitmap.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/drivers/md/md-bitmap.c b/drivers/md/md-bitmap.c index 27409d05f0532..3e4b7b14995a8 100644 --- a/drivers/md/md-bitmap.c +++ b/drivers/md/md-bitmap.c @@ -787,7 +787,7 @@ static int md_bitmap_new_disk_sb(struct bitmap *bitmap) * is a good choice? We choose COUNTER_MAX / 2 arbitrarily. */ write_behind = bitmap->mddev->bitmap_info.max_write_behind; - if (write_behind > COUNTER_MAX) + if (write_behind > COUNTER_MAX / 2) write_behind = COUNTER_MAX / 2; sb->write_behind = cpu_to_le32(write_behind); bitmap->mddev->bitmap_info.max_write_behind = write_behind;