[PATCH] btrfs: qgroup: fix a wrong length calculation in qgroup_free_reserved_data()

In that function, we round down the start position and round up the
ending position.

But during the calculation of @len, we use "round_up(start + len,
sectorsize)", which is the rounded up end position, not the rounded up
length.

Which results a much larger length, and later we are still using "start
+ len", which is completely incorrect.

Fix it by declaring a local @algined_start and @aligned_len and use them
instead.

Fixes: bc42bda22345 ("btrfs: qgroup: Fix qgroup reserved space underflow by only freeing reserved ranges")
Signed-off-by: Qu Wenruo <[email protected]>
---
 fs/btrfs/qgroup.c | 18 +++++++++++-------
 1 file changed, 11 insertions(+), 7 deletions(-)

diff --git a/fs/btrfs/qgroup.c b/fs/btrfs/qgroup.c
index 210af4d7d4b5..f68b696b4bf7 100644
--- a/fs/btrfs/qgroup.c
+++ b/fs/btrfs/qgroup.c
@@ -4339,12 +4339,13 @@ static int qgroup_free_reserved_data(struct btrfs_inode *inode,
 	struct ulist_node *unode;
 	struct ulist_iterator uiter;
 	struct extent_changeset changeset;
+	const u32 sectorsize = root->fs_info->sectorsize;
+	const u64 aligned_start = round_down(start, sectorsize);
+	const u64 aligned_len = round_up(start + len, sectorsize) - aligned_start;
 	u64 freed = 0;
 	int ret;
 
 	extent_changeset_init_bytes_only(&changeset);
-	len = round_up(start + len, root->fs_info->sectorsize);
-	start = round_down(start, root->fs_info->sectorsize);
 
 	ULIST_ITER_INIT(&uiter);
 	while ((unode = ulist_next(&reserved->range_changed, &uiter))) {
@@ -4356,12 +4357,15 @@ static int qgroup_free_reserved_data(struct btrfs_inode *inode,
 
 		extent_changeset_release(&changeset);
 
-		/* Only free range in range [start, start + len) */
-		if (range_start >= start + len ||
-		    range_start + range_len <= start)
+		/*
+		 * Only free the range within
+		 * [aligned_start, aligned_start + aligned_len).
+		 */
+		if (range_start >= aligned_start + aligned_len ||
+		    range_start + range_len <= aligned_start)
 			continue;
-		free_start = max(range_start, start);
-		free_len = min(start + len, range_start + range_len) -
+		free_start = max(range_start, aligned_start);
+		free_len = min(aligned_start + aligned_len, range_start + range_len) -
 			   free_start;
 		/*
 		 * TODO: To also modify reserved->ranges_reserved to reflect
-- 
2.54.0