[PATCH 2/6] btrfs: fix raid stripe search missing entries at leaf boundaries

[robbieko <robbieko@synology.com>]

In btrfs_delete_raid_extent(), the search key uses offset=0. When the
target stripe entry is the first item on a leaf, btrfs_search_slot()
may land on the previous leaf and decrementing the slot from nritems
still points to the wrong entry, causing the stripe extent to be
silently missed.

Fix this by searching with offset=(u64)-1 instead. Since no real stripe
entry has this offset, btrfs_search_slot() always returns 1 with the
slot pointing past the last matching objectid entry. Then unconditionally
decrement the slot with a proper slots[0]==0 early-exit check to handle
the case where no matching entry exists.

Signed-off-by: robbieko <robbieko@synology.com>
---
 fs/btrfs/raid-stripe-tree.c | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/fs/btrfs/raid-stripe-tree.c b/fs/btrfs/raid-stripe-tree.c
index 5c519e161331..d35efe74aa1b 100644
--- a/fs/btrfs/raid-stripe-tree.c
+++ b/fs/btrfs/raid-stripe-tree.c
@@ -98,14 +98,26 @@ int btrfs_delete_raid_extent(struct btrfs_trans_handle *trans, u64 start, u64 le
 	while (1) {
 		key.objectid = start;
 		key.type = BTRFS_RAID_STRIPE_KEY;
-		key.offset = 0;
+		key.offset = (u64)-1;
 
 		ret = btrfs_search_slot(trans, stripe_root, &key, path, -1, 1);
 		if (ret < 0)
 			break;
 
-		if (path->slots[0] == btrfs_header_nritems(path->nodes[0]))
-			path->slots[0]--;
+		/*
+		 * Search with offset=(u64)-1 ensures we land on the correct
+		 * leaf even when the target entry is the first item on a leaf.
+		 * Since no real entry has offset=(u64)-1, ret is always 1 and
+		 * slot points past the last entry with objectid==start (or
+		 * past the end of the leaf if that entry is the last item).
+		 * Back up one slot to find the actual entry.
+		 */
+		if (path->slots[0] == 0) {
+			/* No entry with objectid <= start exists. */
+			ret = 0;
+			break;
+		}
+		path->slots[0]--;
 
 		leaf = path->nodes[0];
 		slot = path->slots[0];
-- 
2.43.0


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