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

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Date:   Fri, 27 Jul 2018 09:34:33 +0000
From:   Vakul Garg <vakul.garg@....com>
To:     Dave Watson <davejwatson@...com>
CC:     David Miller <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "borisp@...lanox.com" <borisp@...lanox.com>,
        "aviadye@...lanox.com" <aviadye@...lanox.com>,
        Doron Roberts-Kedes <doronrk@...com>
Subject: RE: [net-next v5 3/3] net/tls: Remove redundant array allocation.



> -----Original Message-----
> From: Dave Watson [mailto:davejwatson@...com]
> Sent: Thursday, July 26, 2018 2:31 AM
> To: Vakul Garg <vakul.garg@....com>
> Cc: David Miller <davem@...emloft.net>; 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 07/24/18 08:22 AM, Vakul Garg wrote:
> > > 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.
> >
> > Thanks for explaining.
> > I am missing the point why MAX_SKB_FRAGS sized local array sgin has
> > been used in tls_sw_recvmsg(). What is special about MAX_SKB_FRAGS so
> > that we used it as a size factor for 'sgin'?
> 
> There is nothing special about it, in the zerocopy-fastpath if we happen to fit
> in MAX_SKB_FRAGS we avoid any kmalloc whatsoever though.
> It could be renamed MAX_SC_FOR_FASTPATH or something.
> 
> > Will it be a bad idea to get rid of array 'sgin' on stack and simply
> > kmalloc 'sgin' for whatever the number the number of pages returned by
> iov_iter_npages()?
> > We can allocate for sgout too with the same kmalloc().
> >
> > (Using a local array based 'sgin' is coming in the way to achieve
> > sending multiple async record decryption requests to the accelerator
> > without waiting for previous one to complete.)
> 
> Yes we could do this, and yes we would need to heap allocate if you want to
> support multiple outstanding decryption requests.  I think async crypto
> prevents any sort of zerocopy-fastpath, however.

We already do a aead_request_alloc (which internally does kmalloc).
To mitigate the cost of kmalloc/kfree for sg lists and aad, I am allocating a 
combined memory chunk for all of these and then segmenting it into
aead_req, aad, sgin, sgout. This way there should be no extra cost for
memory allocations in non-async.