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From: Vasily Averin <vvs@virtuozzo.com>
To: Christoph Paasch <christoph.paasch@gmail.com>
Cc: "David S. Miller" <davem@davemloft.net>,
	Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
	David Ahern <dsahern@kernel.org>,
	Jakub Kicinski <kuba@kernel.org>,
	Eric Dumazet <eric.dumazet@gmail.com>,
	netdev <netdev@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	kernel@openvz.org, Julian Wiedmann <jwi@linux.ibm.com>
Subject: Re: [PATCH NET v4 3/7] ipv6: use skb_expand_head in ip6_xmit
Date: Mon, 23 Aug 2021 08:59:58 +0300	[thread overview]
Message-ID: <6858f130-e6b4-1ba7-ed6f-58c00152be69@virtuozzo.com> (raw)
In-Reply-To: <7d71f2cc-3fff-beff-b82b-d0a81fb60429@virtuozzo.com>

On 8/23/21 8:44 AM, Vasily Averin wrote:
> On 8/22/21 8:13 PM, Christoph Paasch wrote:
>> On Sun, Aug 22, 2021 at 10:04 AM Christoph Paasch
>> <christoph.paasch@gmail.com> wrote:
>>>
>>> Hello Vasily,
>>>
>>> On Fri, Aug 20, 2021 at 11:21 PM Vasily Averin <vvs@virtuozzo.com> wrote:
>>>>
>>>> On 8/21/21 1:44 AM, Christoph Paasch wrote:
>>>>> (resend without html - thanks gmail web-interface...)
>>>>> On Fri, Aug 20, 2021 at 3:41 PM Christoph Paasch
>>>>>> AFAICS, this is because pskb_expand_head (called from
>>>>>> skb_expand_head) is not adjusting skb->truesize when skb->sk is set
>>>>>> (which I guess is the case in this particular scenario). I'm not
>>>>>> sure what the proper fix would be though...
>>>>
>>>> Could you please elaborate?
>>>> it seems to me skb_realloc_headroom used before my patch called pskb_expand_head() too
>>>> and did not adjusted skb->truesize too. Am I missed something perhaps?
>>>>
>>>> The only difference in my patch is that skb_clone can be not called,
>>>> though I do not understand how this can affect skb->truesize.
>>>
>>> I *believe* that the difference is that after skb_clone() skb->sk is
>>> NULL and thus truesize will be adjusted.
>>>
>>> I will try to confirm that with some more debugging.
>>
>> Yes indeed.
>>
>> Before your patch:
>> [   19.154039] ip6_xmit before realloc truesize 4864 sk? 000000002ccd6868
>> [   19.155230] ip6_xmit after realloc truesize 5376 sk? 0000000000000000
>>
>> skb->sk is not set and thus truesize will be adjusted.
> 
> This looks strange for me. skb should not lost sk reference.
> 
> Could you please clarify where exactly you cheked it?
> sk on newly allocated skb is set on line 291
> 
> net/ipv6/ip6_output.c::ip6_xmit()
>  282         if (unlikely(skb_headroom(skb) < head_room)) {
>  283                 struct sk_buff *skb2 = skb_realloc_headroom(skb, head_room);
>  284                 if (!skb2) {
>  285                         IP6_INC_STATS(net, ip6_dst_idev(skb_dst(skb)),
>  286                                       IPSTATS_MIB_OUTDISCARDS);
>  287                         kfree_skb(skb);
>  288                         return -ENOBUFS;
>  289                 }
>  290                 if (skb->sk)
>  291                         skb_set_owner_w(skb2, skb->sk); <<<<< here
>  292                 consume_skb(skb);
>  293                 skb = skb2;
>  294         }
> 
>> With your change:
>> [   15.092933] ip6_xmit before realloc truesize 4864 sk? 00000000072930fd
>> [   15.094131] ip6_xmit after realloc truesize 4864 sk? 00000000072930fd
>>
>> skb->sk is set and thus truesize is not adjusted.
> 
> In this case skb_set_owner_w() is called inside skb_expand_head()
> 
> net/ipv6/ip6_output.c::ip6_xmit()
>  265         if (unlikely(head_room > skb_headroom(skb))) {
>  266                 skb = skb_expand_head(skb, head_room);
>  267                 if (!skb) {
>  268                         IP6_INC_STATS(net, idev, IPSTATS_MIB_OUTDISCARDS);
>  269                         return -ENOBUFS;
>  270                 }
>  271         }
> 
> net/core/skbuff.c::skb_expand_head()
> 1813         if (skb_shared(skb)) {
> 1814                 struct sk_buff *nskb = skb_clone(skb, GFP_ATOMIC);
> 1815 
> 1816                 if (likely(nskb)) {
> 1817                         if (skb->sk)
> 1818                                 skb_set_owner_w(nskb, skb->sk);  <<<< here
> 1819                         consume_skb(skb);
> 1820                 } else {
> 1821                         kfree_skb(skb);
> 1822                 }
> 1823                 skb = nskb;
> 1824         }
> 
> So I do not understand how this can happen.
> With my patch: 
> a) if skb is not shared -- it should keep original skb->sk
> b) if skb is shared -- new skb should set sk if it was set on original skb.
> 
> Your results can be explained if you looked and skb->sk and truesize right after skb_realloc_headroom() call
> but  before following skb_set_owner_w(). Could you please check it?

It seems I've found the reason:
before my change pskb_expand_head() is called for newly cloned skb where sk was not set.
after my change skb->sk is set before following pskb_expand_head() call

On own turn pskb_expand_head() adjust truesize:

net/core/skbuff.c::pskb_expand_head()
1751         /* It is not generally safe to change skb->truesize.
1752          * For the moment, we really care of rx path, or
1753          * when skb is orphaned (not attached to a socket).
1754          */
1755         if (!skb->sk || skb->destructor == sock_edemux)
1756                 skb->truesize += size - osize;
1757 
1758         return 0;

Could you please confirm it?

Thank you,
	Vasily Averin

  reply	other threads:[~2021-08-23  6:00 UTC|newest]

Thread overview: 106+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <cover.1625665132.git.vvs@virtuozzo.com>
2021-07-07 14:04 ` [PATCH IPV6 1/1] ipv6: allocate enough headroom in ip6_finish_output2() Vasily Averin
2021-07-07 14:45   ` David Ahern
2021-07-07 16:42     ` Jakub Kicinski
2021-07-07 17:41       ` Eric Dumazet
2021-07-07 17:53         ` Vasily Averin
2021-07-07 18:30         ` Jakub Kicinski
2021-07-07 18:50           ` Eric Dumazet
2021-07-09  9:04         ` [PATCH IPV6 v2 0/4] " Vasily Averin
2021-07-12  6:44           ` [PATCH IPV6 v3 0/1] " Vasily Averin
     [not found]           ` <cover.1626069562.git.vvs@virtuozzo.com>
2021-07-12  6:45             ` [PATCH IPV6 v3 1/1] " Vasily Averin
2021-07-12 18:30               ` patchwork-bot+netdevbpf
2021-07-13  7:46               ` Vasily Averin
2021-07-13 12:01                 ` [PATCH NET v4 0/1] " Vasily Averin
     [not found]                 ` <cover.1626177047.git.vvs@virtuozzo.com>
2021-07-13 12:01                   ` [PATCH NET v4 1/1] " Vasily Averin
2021-07-18 10:44                     ` Vasily Averin
2021-07-18 15:22                       ` David Ahern
2021-07-18 17:04                       ` David Miller
2021-07-19  7:55                         ` [PATCH NET] ipv6: ip6_finish_output2: set sk into newly allocated nskb Vasily Averin
2021-07-20 10:10                           ` patchwork-bot+netdevbpf
2021-07-13 12:31                 ` [PATCH IPV6 v3 1/1] ipv6: allocate enough headroom in ip6_finish_output2() Vasily Averin
2021-07-12 13:26           ` [PATCH NET 0/7] skbuff: introduce pskb_realloc_headroom() Vasily Averin
     [not found]           ` <cover.1626093470.git.vvs@virtuozzo.com>
2021-07-12 13:26             ` [PATCH NET 1/7] " Vasily Averin
2021-07-12 17:53               ` Jakub Kicinski
2021-07-12 18:45                 ` Vasily Averin
2021-07-13 20:57                   ` [PATCH NET v2 0/7] skbuff: introduce skb_expand_head() Vasily Averin
2021-08-02  8:52                     ` [PATCH NET v3 " Vasily Averin
     [not found]                     ` <cover.1627891754.git.vvs@virtuozzo.com>
2021-08-02  8:52                       ` [PATCH NET v3 1/7] " Vasily Averin
2021-08-02  8:52                       ` [PATCH NET v3 2/7] ipv6: use skb_expand_head in ip6_finish_output2 Vasily Averin
2021-08-02  8:52                       ` [PATCH NET v3 3/7] ipv6: use skb_expand_head in ip6_xmit Vasily Averin
2021-08-02  8:52                       ` [PATCH NET v3 4/7] ipv4: use skb_expand_head in ip_finish_output2 Vasily Averin
2021-08-02  8:52                       ` [PATCH NET v3 5/7] vrf: use skb_expand_head in vrf_finish_output Vasily Averin
2021-08-05 11:55                         ` Julian Wiedmann
2021-08-05 12:55                           ` Vasily Averin
2021-08-06  7:49                           ` [PATCH NET v4 0/7] skbuff: introduce skb_expand_head() Vasily Averin
2021-08-06 10:14                             ` David Miller
2021-08-06 12:53                               ` [PATCH NET] vrf: fix null pointer dereference in vrf_finish_output() Vasily Averin
2021-08-06 22:42                                 ` Jakub Kicinski
2021-08-07  6:41                                   ` Vasily Averin
     [not found]                           ` <cover.1628235065.git.vvs@virtuozzo.com>
2021-08-06  7:49                             ` [PATCH NET v4 1/7] skbuff: introduce skb_expand_head() Vasily Averin
2021-08-06  7:50                             ` [PATCH NET v4 2/7] ipv6: use skb_expand_head in ip6_finish_output2 Vasily Averin
2021-08-06  7:50                             ` [PATCH NET v4 3/7] ipv6: use skb_expand_head in ip6_xmit Vasily Averin
     [not found]                               ` <CALMXkpaay1y=0tkbnskr4gf-HTMjJJsVryh4Prnej_ws-hJvBg@mail.gmail.com>
2021-08-20 22:44                                 ` Christoph Paasch
2021-08-21  6:21                                   ` Vasily Averin
2021-08-22 17:04                                     ` Christoph Paasch
2021-08-22 17:13                                       ` Christoph Paasch
2021-08-23  5:44                                         ` Vasily Averin
2021-08-23  5:59                                           ` Vasily Averin [this message]
2021-08-23  7:56                                             ` [PATCH NET-NEXT] ipv6: skb_expand_head() adjust skb->truesize incorrectly Vasily Averin
2021-08-23 17:25                                               ` Christoph Paasch
2021-08-23 21:45                                                 ` Eric Dumazet
2021-08-23 21:51                                                   ` Eric Dumazet
2021-08-23 22:23                                                     ` Eric Dumazet
2021-08-24  8:50                                                       ` Vasily Averin
2021-08-24 17:21                                                         ` Vasily Averin
2021-08-25 17:49                                                           ` Christoph Paasch
2021-08-29 12:59                                                             ` [PATCH v2] " Vasily Averin
2021-08-30  5:52                                                               ` [PATCH net-next " Vasily Averin
2021-08-30 16:01                                                               ` [PATCH " Eric Dumazet
2021-08-30 18:09                                                                 ` Vasily Averin
2021-08-30 18:37                                                                   ` Vasily Averin
2021-08-30 19:58                                                                   ` Eric Dumazet
2021-08-31 14:34                                                                     ` [PATCH net-next v3 RFC] " Vasily Averin
2021-08-31 19:38                                                                       ` Eric Dumazet
2021-09-01  6:20                                                                         ` Vasily Averin
2021-09-01  8:11                                                                           ` [PATCH net-next v4] " Vasily Averin
2021-09-01 16:58                                                                             ` Christoph Paasch
2021-09-01 19:17                                                                             ` Eric Dumazet
2021-09-02  3:59                                                                               ` Vasily Averin
2021-09-02  4:32                                                                                 ` Eric Dumazet
2021-09-02  4:48                                                                                   ` Eric Dumazet
2021-09-02  7:13                                                                                     ` Vasily Averin
2021-09-02  7:33                                                                                       ` Vasily Averin
2021-09-02  8:31                                                                                         ` Vasily Averin
2021-09-02 11:12                                                                                           ` [PATCH net-next v5] " Vasily Averin
2021-09-02 15:53                                                                                             ` Christoph Paasch
2021-09-02 16:32                                                                                               ` Vasily Averin
2021-09-06 18:01                                                                                                 ` [PATCH net v6] " Vasily Averin
2021-09-06 18:03                                                                                                   ` Vasily Averin
2021-08-27 15:23                                                       ` [PATCH NET-NEXT] ipv6: " Vasily Averin
2021-08-27 16:47                                                         ` Eric Dumazet
2021-08-28  8:01                                                           ` Vasily Averin
2021-08-06  7:50                             ` [PATCH NET v4 4/7] ipv4: use skb_expand_head in ip_finish_output2 Vasily Averin
2021-08-06  7:50                             ` [PATCH NET v4 5/7] vrf: use skb_expand_head in vrf_finish_output Vasily Averin
2021-08-06  7:50                             ` [PATCH NET v4 6/7] ax25: use skb_expand_head Vasily Averin
2021-08-06  7:50                             ` [PATCH NET v4 7/7] bpf: use skb_expand_head in bpf_out_neigh_v4/6 Vasily Averin
2021-08-02  8:52                       ` [PATCH NET v3 6/7] ax25: use skb_expand_head Vasily Averin
2021-08-02  8:52                       ` [PATCH NET v3 7/7] bpf: use skb_expand_head in bpf_out_neigh_v4/6 Vasily Averin
     [not found]                   ` <cover.1626206993.git.vvs@virtuozzo.com>
2021-07-13 20:57                     ` [PATCH NET v2 1/7] skbuff: introduce skb_expand_head() Vasily Averin
2021-07-13 20:58                     ` [PATCH NET v2 2/7] ipv6: use skb_expand_head in ip6_finish_output2 Vasily Averin
2021-07-13 20:58                     ` [PATCH NET v2 3/7] ipv6: use skb_expand_head in ip6_xmit Vasily Averin
2021-07-13 20:58                     ` [PATCH NET v2 4/7] ipv4: use skb_expand_head in ip_finish_output2 Vasily Averin
2021-07-13 20:58                     ` [PATCH NET v2 5/7] vrf: use skb_expand_head in vrf_finish_output Vasily Averin
2021-07-13 20:58                     ` [PATCH NET v2 6/7] ax25: use skb_expand_head Vasily Averin
2021-07-13 20:58                     ` [PATCH NET v2 7/7] bpf: use skb_expand_head in bpf_out_neigh_v4/6 Vasily Averin
2021-07-12 13:26             ` [PATCH NET 2/7] ipv6: use pskb_realloc_headroom in ip6_finish_output2 Vasily Averin
2021-07-12 13:26             ` [PATCH NET 3/7] ipv6: use pskb_realloc_headroom in ip6_xmit refactoring Vasily Averin
2021-07-12 13:27             ` [PATCH NET 4/7] ipv4: use pskb_realloc_headroom in ip_finish_output2 Vasily Averin
2021-07-12 13:27             ` [PATCH NET 5/7] vrf: use pskb_realloc_headroom in vrf_finish_output Vasily Averin
2021-07-12 13:27             ` [PATCH NET 6/7] ax25: use pskb_realloc_headroom Vasily Averin
2021-07-12 13:27             ` [PATCH NET 7/7] bpf: use pskb_realloc_headroom in bpf_out_neigh_v4/6 Vasily Averin
     [not found]         ` <cover.1625818825.git.vvs@virtuozzo.com>
2021-07-09  9:04           ` [PATCH IPV6 v2 1/4] ipv6: allocate enough headroom in ip6_finish_output2() Vasily Averin
2021-07-09 17:58             ` David Miller
2021-07-10  2:53               ` Vasily Averin
2021-07-09  9:04           ` [PATCH IPV6 v2 2/4] ipv6: use new helper skb_expand_head() in ip6_xmit() Vasily Averin
2021-07-09  9:05           ` [PATCH IPV6 v2 3/4] ipv6: ip6_finish_output2 refactoring Vasily Averin
2021-07-09  9:05           ` [PATCH IPV6 v2 4/4] ipv6: ip6_xmit refactoring Vasily Averin

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