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Commit 2871c69e authored by Joe Thornber's avatar Joe Thornber Committed by Mike Snitzer
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dm btree remove: fix a bug when rebalancing nodes after removal 

Commit 4c7e3093 ("dm btree remove: fix bug in redistribute3") wasn't
a complete fix for redistribute3().

The redistribute3 function takes 3 btree nodes and shares out the entries
evenly between them.  If the three nodes in total contained
(MAX_ENTRIES * 3) - 1 entries between them then this was erroneously getting
rebalanced as (MAX_ENTRIES - 1) on the left and right, and (MAX_ENTRIES + 1) in
the center.

Fix this issue by being more careful about calculating the target number
of entries for the left and right nodes.

Unit tested in userspace using this program:
https://github.com/jthornber/redistribute3-test/blob/master/redistribute3_t.c



Signed-off-by: default avatarJoe Thornber <ejt@redhat.com>
Signed-off-by: default avatarMike Snitzer <snitzer@redhat.com>
Cc: stable@vger.kernel.org
parent ba30670f

drivers/md/persistent-data/dm-btree-remove.c

+11 −6
Original line number Diff line number Diff line
@@ -301,11 +301,16 @@ static void redistribute3(struct dm_btree_info *info, struct btree_node *parent, 
{

	int s;

	uint32_t max_entries = le32_to_cpu(left->header.max_entries);

	unsigned target = (nr_left + nr_center + nr_right) / 3;

	BUG_ON(target > max_entries);

	unsigned total = nr_left + nr_center + nr_right;

	unsigned target_right = total / 3;

	unsigned remainder = (target_right * 3) != total;

	unsigned target_left = target_right + remainder;



	BUG_ON(target_left > max_entries);

	BUG_ON(target_right > max_entries);



	if (nr_left < nr_right) {

		s = nr_left - target;

		s = nr_left - target_left;



		if (s < 0 && nr_center < -s) {

			/* not enough in central node */
@@ -316,10 +321,10 @@ static void redistribute3(struct dm_btree_info *info, struct btree_node *parent, 
		} else

			shift(left, center, s);



		shift(center, right, target - nr_right);

		shift(center, right, target_right - nr_right);



	} else {

		s = target - nr_right;

		s = target_right - nr_right;

		if (s > 0 && nr_center < s) {

			/* not enough in central node */

			shift(center, right, nr_center);
@@ -329,7 +334,7 @@ static void redistribute3(struct dm_btree_info *info, struct btree_node *parent, 
		} else

			shift(center, right, s);



		shift(left, center, nr_left - target);

		shift(left, center, nr_left - target_left);

	}



	*key_ptr(parent, c->index) = center->keys[0];