[dm-devel] kernel BUG at drivers/md/persistent-data/dm-btree-remove.c:182!
Joe Thornber
thornber at redhat.com
Wed Oct 21 17:49:40 UTC 2015
On Tue, Oct 20, 2015 at 10:39:31AM +0800, Dennis Yang wrote:
> Hi,
>
> After I analyzed the metadata of this case couple months ago, I find
> out that there is another possible bug which might trigger this
> assertion fail in shift(). I had posted a patch two months ago on the
> list to explain and fix this issue. Could you help reviewing this?
>
> https://www.redhat.com/archives/dm-devel/2015-August/msg00155.html
Yep, that's a bug. Sorry I missed it before when you posted. Here's
my fix:
commit 05ce4edf20c7e4a0d7a3c8d87a3d4b6744d0fea2
Author: Joe Thornber <ejt at redhat.com>
Date: Wed Oct 21 18:36:49 2015 +0100
[dm-btree] Fix bug when rebalancing nodes after removal
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.
This patch is more careful about calculating the target nr 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
diff --git a/drivers/md/persistent-data/dm-btree-remove.c b/drivers/md/persistent-data/dm-btree-remove.c
index 4222f77..1dac15d 100644
--- 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_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];
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