| From: Joe Thornber <ejt@redhat.com> |
| Date: Wed, 21 Oct 2015 18:36:49 +0100 |
| Subject: dm btree remove: fix a bug when rebalancing nodes after removal |
| |
| commit 2871c69e025e8bc507651d5a9cf81a8a7da9d24b upstream. |
| |
| Commit 4c7e309340ff ("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: Joe Thornber <ejt@redhat.com> |
| Signed-off-by: Mike Snitzer <snitzer@redhat.com> |
| Signed-off-by: Ben Hutchings <ben@decadent.org.uk> |
| --- |
| drivers/md/persistent-data/dm-btree-remove.c | 17 +++++++++++------ |
| 1 file changed, 11 insertions(+), 6 deletions(-) |
| |
| --- a/drivers/md/persistent-data/dm-btree-remove.c |
| +++ b/drivers/md/persistent-data/dm-btree-remove.c |
| @@ -301,11 +301,16 @@ static void redistribute3(struct dm_btre |
| { |
| 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_btre |
| } 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_btre |
| } 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]; |