netdev - Re: [net-next v5 3/3] net/tls: Remove redundant array allocation.

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Date:   Mon, 23 Jul 2018 09:35:09 -0700
From:   Dave Watson <davejwatson@...com>
To:     David Miller <davem@...emloft.net>
CC:     <vakul.garg@....com>, <netdev@...r.kernel.org>,
        <borisp@...lanox.com>, <aviadye@...lanox.com>,
        Doron Roberts-Kedes <doronrk@...com>
Subject: Re: [net-next v5 3/3] net/tls: Remove redundant array allocation.

On 07/21/18 07:25 PM, David Miller wrote:
> From: Vakul Garg <vakul.garg@....com>
> Date: Thu, 19 Jul 2018 21:56:13 +0530
> 
> > In function decrypt_skb(), array allocation in case when sgout is NULL
> > is unnecessary. Instead, local variable sgin_arr[] can be used.
> > 
> > Signed-off-by: Vakul Garg <vakul.garg@....com>
> 
> Hmmm...
> 
> Dave, can you take a look at this?  Do you think there might have
> been a reason you felt that you needed to dynamically allocate
> the scatterlists when you COW and skb and do in-place decryption?
> 
> I guess this change is ok, nsg can only get smaller when the SKB
> is COW'd.
> 

> >         memcpy(iv, tls_ctx->rx.iv, TLS_CIPHER_AES_GCM_128_SALT_SIZE);
> >         if (!sgout) {
> >                 nsg = skb_cow_data(skb, 0, &unused) + 1;
> > -               sgin = kmalloc_array(nsg, sizeof(*sgin), sk->sk_allocation);
> >                 sgout = sgin;
> >         }

I don't think this patch is safe as-is.  sgin_arr is a stack array of
size MAX_SKB_FRAGS (+ overhead), while my read of skb_cow_data is that
it walks the whole chain of skbs from skb->next, and can return any
number of segments.  Therefore we need to heap allocate.  I think I
copied the IPSEC code here.

For perf though, we could use the stack array if skb_cow_data returns
<= MAX_SKB_FRAGS.

This code is slightly confusing though, since we don't heap allocate
in the zerocopy case - what happens is that skb_to_sgvec returns
-EMSGSIZE, and we fall back to the non-zerocopy case, and return again
to this function, where we then hit the skb_cow_data path and heap
allocate.