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

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Date:   Fri, 14 Oct 2016 10:35:56 +0100
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Johannes Berg <johannes@...solutions.net>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        "linux-next@...r.kernel.org" <linux-next@...r.kernel.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Network Development <netdev@...r.kernel.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>,
        Linux Wireless List <linux-wireless@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Jouni Malinen <j@...fi>
Subject: Re: [mac80211] BUG_ON with current -git (4.8.0-11417-g24532f7)

On 14 October 2016 at 10:25, Johannes Berg <johannes@...solutions.net> 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