Subject: Re: [net-next PATCH 5/9] fib_trie: Add tnode struct as a container for fields not needed in key_vector

From: Alexander Duyck <[email-masked]>
Date: Fri, 06 Mar 2015 08:13:35 -0800

On 03/06/2015 05:17 AM, Sergei Shtylyov wrote:
> Hello.
>
> On 3/6/2015 1:51 AM, Alexander Duyck wrote:
>
>> This change pulls the fields not explicitly needed in the key_vector and
>> placed them in the new tnode structure.  By doing this we will
>> eventually
>> be able to reduce the key_vector down to 16 bytes on 64 bit systems, and
>> 12 bytes on 32 bit systems.
>
>> Signed-off-by: Alexander Duyck <[email-masked]>
>> ---
>>   net/ipv4/fib_trie.c |   72
>> ++++++++++++++++++++++++++++-----------------------
>>   1 file changed, 39 insertions(+), 33 deletions(-)
>>
>> diff --git a/net/ipv4/fib_trie.c b/net/ipv4/fib_trie.c
>> index f06c92e..5e1c469 100644
>> --- a/net/ipv4/fib_trie.c
>> +++ b/net/ipv4/fib_trie.c
> [...]
>> @@ -316,48 +320,50 @@ static inline void empty_child_dec(struct
>> key_vector *n)
>>
>>   static struct key_vector *leaf_new(t_key key, struct fib_alias *fa)
>>   {
> [...]
>> +    struct tnode *kv = kmem_cache_alloc(trie_leaf_kmem, GFP_KERNEL);
>> +    struct key_vector *l = kv->kv;
>> +
>> +    if (!kv)
>
>    You dereference 'kv' before this check.
>
> [...]
>>   static struct key_vector *tnode_new(t_key key, int pos, int bits)
>>   {
>> -    struct key_vector *tn = tnode_alloc(bits);
>> +    struct tnode *tnode = tnode_alloc(bits);
>>       unsigned int shift = pos + bits;
>> +    struct key_vector *tn = tnode->kv;
>>
>>       /* verify bits and pos their msb bits clear and values are
>> valid */
>>       BUG_ON(!bits || (shift > KEYLENGTH));
>>
> [...]
>> +    pr_debug("AT %p s=%zu %zu\n", tnode, TNODE_SIZE(0),
>>            sizeof(struct key_vector *) << bits);
>> +
>> +    if (!tnode)
>
>    Likewise with 'tnode'.
>
> [...]
>
> WBR, Sergei

Actually neither of these are a deference.  The 'kv' member is an array
of size 1, so tnode->kv is actually just adding the offset of the array
to the pointer and storing it.  So if tnode is NULL, then tnode->kv is
NULL + 32 on a 64b system.  It isn't really a dereference until I use
the ->, *, or [] operators on the tn pointer.

- Alex