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From: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
To: dev@dpdk.org
Cc: Morten Brørup <mb@smartsharesystems.com>,
	"Ananyev, Konstantin" <konstantin.ananyev@intel.com>,
	dpdk stable <stable@dpdk.org>,
	Olivier Matz <olivier.matz@6wind.com>
Subject: Re: [dpdk-stable] [dpdk-dev] [PATCH] mbuf: fix reset on mbuf free
Date: Thu, 5 Nov 2020 12:09:23 +0300
Message-ID: <1f1b16c6-09d3-f020-ee7b-10ac6536554e@oktetlabs.ru> (raw)
In-Reply-To: <20201105074626.GL1898@platinum>

Just resend with lost Cc restored.

On 11/5/20 10:46 AM, Olivier Matz wrote:
> On Thu, Nov 05, 2020 at 12:15:49AM +0000, Ananyev, Konstantin wrote:
>>
>> Hi Olivier,
>>  
>>> m->nb_seg must be reset on mbuf free whatever the value of m->next,
>>> because it can happen that m->nb_seg is != 1. For instance in this
>>> case:
>>>
>>>   m1 = rte_pktmbuf_alloc(mp);
>>>   rte_pktmbuf_append(m1, 500);
>>>   m2 = rte_pktmbuf_alloc(mp);
>>>   rte_pktmbuf_append(m2, 500);
>>>   rte_pktmbuf_chain(m1, m2);
>>>   m0 = rte_pktmbuf_alloc(mp);
>>>   rte_pktmbuf_append(m0, 500);
>>>   rte_pktmbuf_chain(m0, m1);
>>>
>>> As rte_pktmbuf_chain() does not reset nb_seg in the initial m1
>>> segment (this is not required), after this code the mbuf chain
>>> have 3 segments:
>>>   - m0: next=m1, nb_seg=3
>>>   - m1: next=m2, nb_seg=2
>>>   - m2: next=NULL, nb_seg=1
>>>
>>> Freeing this mbuf chain will not restore nb_seg=1 in the second
>>> segment. 
>>
>> Hmm, not sure why is that?
>> You are talking about freeing m1, right?
>> rte_pktmbuf_prefree_seg(struct rte_mbuf *m)
>> {
>> 	...
>> 	if (m->next != NULL) {
>>                         m->next = NULL;
>>                         m->nb_segs = 1;
>>                 }
>>
>> m1->next != NULL, so it will enter the if() block,
>> and will reset both next and nb_segs.
>> What I am missing here? 
>> Thinking in more generic way, that change:
>>  -		if (m->next != NULL) {
>>  -			m->next = NULL;
>>  -			m->nb_segs = 1;
>>  -		}
>>  +		m->next = NULL;
>>  +		m->nb_segs = 1;
> 
> Ah, sorry. I oversimplified the example and now it does not
> show the issue...
> 
> The full example also adds a split() to break the mbuf chain
> between m1 and m2. The kind of thing that would be done for
> software TCP segmentation.
> 

If so, may be the right solution is to care about nb_segs
when next is set to NULL on split? Any place when next is set
to NULL. Just to keep the optimization in a more generic place.

> After this operation, we have 2 mbuf chain:
>  - m0 with 2 segments, the last one has next=NULL but nb_seg=2
>  - new_m with 1 segment
> 
> Freeing m0 will not restore nb_seg=1 in the second segment.
> 
>> Assumes that it is ok to have an mbuf with
>> nb_seg > 1 and next == NULL.
>> Which seems wrong to me.
> 
> I don't think it is wrong: nb_seg is just ignored when not in the first
> segment, and there is nothing saying it should be set to 1. Typically,
> rte_pktmbuf_chain() does not change it, and I guess it's the same for
> many similar functions in applications.
> 
> Olivier
> 
>>
>>
>>> This is expected that mbufs stored in pool have their
>>> nb_seg field set to 1.
>>>
>>> Fixes: 8f094a9ac5d7 ("mbuf: set mbuf fields while in pool")
>>> Cc: stable@dpdk.org
>>>
>>> Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
>>> ---
>>>  lib/librte_mbuf/rte_mbuf.c |  6 ++----
>>>  lib/librte_mbuf/rte_mbuf.h | 12 ++++--------
>>>  2 files changed, 6 insertions(+), 12 deletions(-)
>>>
>>> diff --git a/lib/librte_mbuf/rte_mbuf.c b/lib/librte_mbuf/rte_mbuf.c
>>> index 8a456e5e64..e632071c23 100644
>>> --- a/lib/librte_mbuf/rte_mbuf.c
>>> +++ b/lib/librte_mbuf/rte_mbuf.c
>>> @@ -129,10 +129,8 @@ rte_pktmbuf_free_pinned_extmem(void *addr, void *opaque)
>>>
>>>  	rte_mbuf_ext_refcnt_set(m->shinfo, 1);
>>>  	m->ol_flags = EXT_ATTACHED_MBUF;
>>> -	if (m->next != NULL) {
>>> -		m->next = NULL;
>>> -		m->nb_segs = 1;
>>> -	}
>>> +	m->next = NULL;
>>> +	m->nb_segs = 1;
>>>  	rte_mbuf_raw_free(m);
>>>  }
>>>
>>> diff --git a/lib/librte_mbuf/rte_mbuf.h b/lib/librte_mbuf/rte_mbuf.h
>>> index a1414ed7cd..ef5800c8ef 100644
>>> --- a/lib/librte_mbuf/rte_mbuf.h
>>> +++ b/lib/librte_mbuf/rte_mbuf.h
>>> @@ -1329,10 +1329,8 @@ rte_pktmbuf_prefree_seg(struct rte_mbuf *m)
>>>  				return NULL;
>>>  		}
>>>
>>> -		if (m->next != NULL) {
>>> -			m->next = NULL;
>>> -			m->nb_segs = 1;
>>> -		}
>>> +		m->next = NULL;
>>> +		m->nb_segs = 1;
>>>
>>>  		return m;
>>>
>>> @@ -1346,10 +1344,8 @@ rte_pktmbuf_prefree_seg(struct rte_mbuf *m)
>>>  				return NULL;
>>>  		}
>>>
>>> -		if (m->next != NULL) {
>>> -			m->next = NULL;
>>> -			m->nb_segs = 1;
>>> -		}
>>> +		m->next = NULL;
>>> +		m->nb_segs = 1;
>>>  		rte_mbuf_refcnt_set(m, 1);
>>>
>>>  		return m;
>>> --
>>> 2.25.1
>>


  parent reply	other threads:[~2020-11-05  9:09 UTC|newest]

Thread overview: 38+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-11-04 17:00 [dpdk-stable] " Olivier Matz
2020-11-05  0:15 ` Ananyev, Konstantin
2020-11-05  7:46   ` Olivier Matz
2020-11-05  8:33     ` [dpdk-stable] [dpdk-dev] " Morten Brørup
2020-11-05  9:03       ` Olivier Matz
2020-11-05  9:09     ` Andrew Rybchenko [this message]
2020-11-08  7:25 ` Ali Alnubani
2020-12-18 12:52 ` [dpdk-stable] [PATCH v2] " Olivier Matz
2020-12-18 13:18   ` [dpdk-stable] [dpdk-dev] " Morten Brørup
2020-12-18 23:33     ` Ajit Khaparde
2021-01-06 13:33 ` [dpdk-stable] [PATCH v3] " Olivier Matz
2021-01-10  9:28   ` Ali Alnubani
2021-01-11 13:14   ` Ananyev, Konstantin
2021-01-13 13:27 ` [dpdk-stable] [PATCH v4] " Olivier Matz
2021-01-15 13:59   ` David Marchand
2021-01-15 18:39     ` Ali Alnubani
2021-01-18 17:52       ` Ali Alnubani
2021-01-19  8:32         ` Olivier Matz
2021-01-19  8:53           ` Morten Brørup
2021-01-19 12:00             ` Ferruh Yigit
2021-01-19 12:27               ` [dpdk-stable] [dpdk-dev] " Morten Brørup
2021-01-19 14:03                 ` Ferruh Yigit
2021-01-19 14:21                   ` Morten Brørup
2021-01-21  9:15                     ` Ferruh Yigit
2021-01-19 14:04           ` [dpdk-stable] " Slava Ovsiienko
2021-07-24  8:47             ` [dpdk-stable] [dpdk-dev] " Thomas Monjalon
2021-07-30 12:36               ` Olivier Matz
2021-07-30 14:35                 ` Morten Brørup
2021-07-30 14:54                   ` Thomas Monjalon
2021-07-30 15:14                     ` Olivier Matz
2021-07-30 15:23                       ` Morten Brørup
2021-01-21  9:19       ` [dpdk-stable] " Ferruh Yigit
2021-01-21  9:29         ` [dpdk-stable] [dpdk-dev] " Morten Brørup
2021-01-21 16:35           ` [dpdk-stable] [dpdklab] " Lincoln Lavoie
2021-01-23  8:57             ` [dpdk-stable] [dpdk-dev] [dpdklab] " Morten Brørup
2021-01-25 17:00               ` Brandon Lo
2021-01-25 18:42             ` [dpdk-stable] [dpdklab] RE: [dpdk-dev] " Ferruh Yigit
2021-06-15 13:56   ` [dpdk-stable] " Morten Brørup

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