On Thu, May 19, 2022 at 09:50:30AM +0100, David Howells wrote:
From: Stephen Brennan stephen.s.brennan@oracle.com
A rare BUG_ON triggered in assoc_array_gc:
[3430308.818153] kernel BUG at lib/assoc_array.c:1609!Which corresponded to the statement currently at line 1593 upstream:
BUG_ON(assoc_array_ptr_is_meta(p));Using the data from the core dump, I was able to generate a userspace reproducer[1] and determine the cause of the bug.
After running the iterator on the entire branch, an internal tree node looked like the following:
NODE (nr_leaves_on_branch: 3) SLOT [0] NODE (2 leaves) SLOT [1] NODE (1 leaf) SLOT [2..f] NODE (empty)In the userspace reproducer, the pr_devel output when compressing this node was:
-- compress node 0x5607cc089380 -- free=0, leaves=0 [0] retain node 2/1 [nx 0] [1] fold node 1/1 [nx 0] [2] fold node 0/1 [nx 2] [3] fold node 0/2 [nx 2] [4] fold node 0/3 [nx 2] [5] fold node 0/4 [nx 2] [6] fold node 0/5 [nx 2] [7] fold node 0/6 [nx 2] [8] fold node 0/7 [nx 2] [9] fold node 0/8 [nx 2] [10] fold node 0/9 [nx 2] [11] fold node 0/10 [nx 2] [12] fold node 0/11 [nx 2] [13] fold node 0/12 [nx 2] [14] fold node 0/13 [nx 2] [15] fold node 0/14 [nx 2] after: 3At slot 0, an internal node with 2 leaves could not be folded into the node, because there was only one available slot (slot 0). Thus, the internal node was retained. At slot 1, the node had one leaf, and was able to be folded in successfully. The remaining nodes had no leaves, and so were removed. By the end of the compression stage, there were 14 free slots, and only 3 leaf nodes. The tree was ascended and then its parent node was compressed. When this node was seen, it could not be folded, due to the internal node it contained.
The invariant for compression in this function is: whenever nr_leaves_on_branch < ASSOC_ARRAY_FAN_OUT, the node should contain all leaf nodes. The compression step currently cannot guarantee this, given the corner case shown above.
To fix this issue, retry compression whenever we have retained a node, and yet nr_leaves_on_branch < ASSOC_ARRAY_FAN_OUT. This second compression will then allow the node in slot 1 to be folded in, satisfying the invariant. Below is the output of the reproducer once the fix is applied:
-- compress node 0x560e9c562380 -- free=0, leaves=0 [0] retain node 2/1 [nx 0] [1] fold node 1/1 [nx 0] [2] fold node 0/1 [nx 2] [3] fold node 0/2 [nx 2] [4] fold node 0/3 [nx 2] [5] fold node 0/4 [nx 2] [6] fold node 0/5 [nx 2] [7] fold node 0/6 [nx 2] [8] fold node 0/7 [nx 2] [9] fold node 0/8 [nx 2] [10] fold node 0/9 [nx 2] [11] fold node 0/10 [nx 2] [12] fold node 0/11 [nx 2] [13] fold node 0/12 [nx 2] [14] fold node 0/13 [nx 2] [15] fold node 0/14 [nx 2] internal nodes remain despite enough space, retrying -- compress node 0x560e9c562380 -- free=14, leaves=1 [0] fold node 2/15 [nx 0] after: 3Changes
DH:
- Use false instead of 0.
- Reorder the inserted lines in a couple of places to put retained before next_slot.
ver #2)
- Fix typo in pr_devel, correct comparison to "<="
Fixes: 3cb989501c26 ("Add a generic associative array implementation.") Cc: stable@vger.kernel.org Signed-off-by: Stephen Brennan stephen.s.brennan@oracle.com Signed-off-by: David Howells dhowells@redhat.com cc: Jarkko Sakkinen jarkko@kernel.org cc: Andrew Morton akpm@linux-foundation.org cc: keyrings@vger.kernel.org Link: https://lore.kernel.org/r/20220511225517.407935-1-stephen.s.brennan@oracle.c... # v1 Link: https://lore.kernel.org/r/20220512215045.489140-1-stephen.s.brennan@oracle.c... # v2
lib/assoc_array.c | 8 ++++++++ 1 file changed, 8 insertions(+)
diff --git a/lib/assoc_array.c b/lib/assoc_array.c index 079c72e26493..ca0b4f360c1a 100644 --- a/lib/assoc_array.c +++ b/lib/assoc_array.c @@ -1461,6 +1461,7 @@ int assoc_array_gc(struct assoc_array *array, struct assoc_array_ptr *cursor, *ptr; struct assoc_array_ptr *new_root, *new_parent, **new_ptr_pp; unsigned long nr_leaves_on_tree;
- bool retained; int keylen, slot, nr_free, next_slot, i;
pr_devel("-->%s()\n", __func__); @@ -1536,6 +1537,7 @@ int assoc_array_gc(struct assoc_array *array, goto descend; } +retry_compress: pr_devel("-- compress node %p --\n", new_n); /* Count up the number of empty slots in this node and work out the @@ -1553,6 +1555,7 @@ int assoc_array_gc(struct assoc_array *array, pr_devel("free=%d, leaves=%lu\n", nr_free, new_n->nr_leaves_on_branch); /* See what we can fold in */
- retained = false; next_slot = 0; for (slot = 0; slot < ASSOC_ARRAY_FAN_OUT; slot++) { struct assoc_array_shortcut *s;
@@ -1602,9 +1605,14 @@ int assoc_array_gc(struct assoc_array *array, pr_devel("[%d] retain node %lu/%d [nx %d]\n", slot, child->nr_leaves_on_branch, nr_free + 1, next_slot);
retained = true;
- if (retained && new_n->nr_leaves_on_branch <= ASSOC_ARRAY_FAN_OUT) {
pr_devel("internal nodes remain despite enough space, retrying\n");goto retry_compress;- } pr_devel("after: %lu\n", new_n->nr_leaves_on_branch);
nr_leaves_on_tree = new_n->nr_leaves_on_branch;
Reviewed-by: Jarkko Sakkinen jarkko@kernel.org
BR, Jarkko