Re: [mac80211] BUG_ON with current -git (4.8.0-11417-g24532f7)

[Ard Biesheuvel]

On 14 October 2016 at 10:25, Johannes Berg <johannes@xxxxxxxxxxxxxxxx> wrote:
> On Fri, 2016-10-14 at 10:21 +0100, Ard Biesheuvel wrote:
>
>> It is annotated with a TODO, though :-)
>>
>> 38320c70d282b (Herbert Xu               2008-01-28 19:35:05
>> -0800  41)
>>  * TODO: Use spare space in skb for this where possible.
>
> I saw that, but I don't think generally there will be spare space for
> it - the stuff there is likely far too big. Anyway ... same problem
> that we have.
>
> I'm not inclined to allocate ~500 bytes temporarily for every frame
> either though.
>
> Maybe we could try to manage it in mac80211, we'd "only" need 5 AEAD
> structs (which are today on the stack) in parallel for each key (4 TX,
> 1 RX), but in a typical case of having 3 keys that's already 7.5K worth
> of memory that we almost never use. Again, with more complexity, we
> could know that the TX will not be used if the driver does the TX, but
> the single RX one we'd need unconditionally... decisions decisions...
>

So why is the performance hit acceptable for ESP but not for WPA? We
could easily implement the same thing, i.e., kmalloc(GFP_ATOMIC)/kfree
the aead_req struct rather than allocate it on the stack