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Date:   Mon, 13 Mar 2017 14:09:23 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Tariq Toukan <tariqt@...lanox.com>,
        Saeed Mahameed <saeedm@...lanox.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Alexander Duyck <alexander.duyck@...il.com>
Subject: Re: [PATCH net-next] mlx4: Better use of order-0 pages in RX path

On Mon, Mar 13, 2017 at 1:23 PM, Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
> On Mon, Mar 13, 2017 at 11:58:05AM -0700, Eric Dumazet wrote:
>> On Mon, Mar 13, 2017 at 11:31 AM, Alexei Starovoitov
>> <alexei.starovoitov@...il.com> wrote:
>> > On Mon, Mar 13, 2017 at 10:50:28AM -0700, Eric Dumazet wrote:
>> >> On Mon, Mar 13, 2017 at 10:34 AM, Alexei Starovoitov
>> >> <alexei.starovoitov@...il.com> wrote:
>> >> > On Sun, Mar 12, 2017 at 05:58:47PM -0700, Eric Dumazet wrote:
>> >> >> @@ -767,10 +814,30 @@ int mlx4_en_process_rx_cq(struct net_device *dev, struct mlx4_en_cq *cq, int bud
>> >> >>                       case XDP_PASS:
>> >> >>                               break;
>> >> >>                       case XDP_TX:
>> >> >> +                             /* Make sure we have one page ready to replace this one */
>> >> >> +                             npage = NULL;
>> >> >> +                             if (!ring->page_cache.index) {
>> >> >> +                                     npage = mlx4_alloc_page(priv, ring,
>> >> >> +                                                             &ndma, numa_mem_id(),
>> >> >> +                                                             GFP_ATOMIC | __GFP_MEMALLOC);
>> >> >
>> >> > did you test this with xdp2 test ?
>> >> > under what conditions it allocates ?
>> >> > It looks dangerous from security point of view to do allocations here.
>> >> > Can it be exploited by an attacker?
>> >> > we use xdp for ddos and lb and this is fast path.
>> >> > If 1 out of 100s XDP_TX packets hit this allocation we will have serious
>> >> > perf regression.
>> >> > In general I dont think it's a good idea to penalize x86 in favor of powerpc.
>> >> > Can you #ifdef this new code somehow? so we won't have these concerns on x86?
>> >>
>> >> Normal paths would never hit this point really. I wanted to be extra
>> >> safe, because who knows, some guys could be tempted to set
>> >> ethtool -G ethX  rx 512 tx 8192
>> >>
>> >> Before this patch, if you were able to push enough frames in TX ring,
>> >> you would also eventually be forced to allocate memory, or drop frames...
>> >
>> > hmm. not following.
>> > Into xdp tx queues packets don't come from stack. It can only be via xdp_tx.
>> > So this rx page belongs to driver, not shared with anyone and it only needs to
>> > be put onto tx ring, so I don't understand why driver needs to allocating
>> > anything here. To refill the rx ring? but why here?
>>
>> Because RX ring can be shared, by packets goind to the upper stacks (say TCP)
>>
>> So there is no guarantee that the pages in the quarantine pool have
>> their page count to 1.
>>
>> The normal TX_XDP path will recycle pages in ring->cache_page .
>>
>> This is exactly where I pick up a replacement.
>>
>> Pages in ring->cache_page have the guarantee to have no other users
>> than ourself (mlx4 driver)
>>
>> You might have not noticed that current mlx4 driver has a lazy refill
>> of RX ring buffers, that eventually
>> removes all the pages from RX ring, and we have to recover with this
>> lazy mlx4_en_recover_from_oom() thing
>> that will attempt to restart the allocations.
>>
>> After my patch, we have the guarantee that the RX ring buffer is
>> always fully populated.
>>
>> When we receive a frame (XDP or not), we drop it if we can not
>> replenish the RX slot,
>> in case the oldest page in quarantine is not a recycling candidate and
>> we can not allocate a new page.
>
> Got it. Could you please add above explanation to the commit log,
> since it's a huge difference vs other drivers.
> I don't think any other driver in the tree follows this strategy.

sk_buff allocation can fail before considering adding a frag on the
skb anyway...

Look, all this is completely irrelevant for XDP_TX, since there is no
allocation at all,
once ~128 pages have been put into page_cache.

Do you want to prefill this cache when XDP is loaded ?

> I think that's the right call, but it shouldn't be hidden in details.

ring->page_cache contains the pages used by XDP_TX are recycled
through mlx4_en_rx_recycle()

There is a nice comment there. I did not change this part.

These pages _used_ to be directly taken from mlx4_en_prepare_rx_desc(),
because there was no page recycling for the normal path.

All my patch does is a generic implementation
for page recycling, leaving the XDP_TX pages in their own pool,
because we do not even have to check their page count, adding a cache line miss.

So I do have 2 different pools, simply to let XDP_TX path being ultra
fast, as before.

Does not look details to me.


> If that's the right approach (probably is) we should do it
> in the other drivers too.
> Though I don't see you dropping "to the stack" packet in this patch.
> The idea is to drop the packet (whether xdp or not) if rx
> buffer cannot be replinshed _regardless_ whether driver
> implements recycling, right?



>
> Theory vs practice. We don't see this issue running xdp.

Sure, so lets ignore syzkaller guys and stop doing code reviews, if
all this is working for you.

> My understanding, since rx and tx napi are scheduled for the same cpu
> there is a guarantee that during normal invocation they won't race
> and the only problem is due to mlx4_en_recover_from_oom() that can
> run rx napi on some random cpu.

Exactly. So under pressure, host will panic :/

Same if you let one application using busy polling, or if IRQ is moved
to another cpu (/proc/irq/*/smp_affinity)

Looks fragile, but whatever, maybe you control everything on your hosts.

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