Re: [PATCH] assoc_array: Fix BUG_ON during garbage collect

[Stephen Brennan <stephen.s.brennan@oracle.com>]

David Howells <dhowells@redhat.com> writes:
> 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.
>
> [1]: https://github.com/brenns10/kernel_stuff/tree/master/assoc_array_gc
>
> 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: 3
>
> At 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: 3
>
> Changes
> =======
> DH:
>  - Use false instead of 0.
>  - Reorder the inserted lines in a couple of places to put retained before
>    next_slot.

Thanks for the fixes, sorry I left so many loose ends on this fix! All
looks good to me.

Stephen

>
> 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.com/ # v1
> Link: https://lore.kernel.org/r/20220512215045.489140-1-stephen.s.brennan@oracle.com/ # 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;