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Date:      Wed, 21 Aug 2013 22:44:07 +0200
From:      Andre Oppermann <andre@freebsd.org>
To:        Navdeep Parhar <np@FreeBSD.org>
Cc:        svn-src-head@freebsd.org, freebsd-net@freebsd.org, Peter Grehan <grehan@freebsd.org>
Subject:   Re: M_NOFREE removal (was Re: svn commit: r254520 - in head/sys: kern sys)
Message-ID:  <52152697.9070500@freebsd.org>
In-Reply-To: <52151799.3020809@FreeBSD.org>
References:  <201308191116.r7JBGsc6065793@svn.freebsd.org> <521256CE.6070706@FreeBSD.org> <5212587A.2080202@freebsd.org> <52128937.1010407@freebsd.org> <52129E55.30803@freebsd.org> <5214D7E8.1080106@freebsd.org> <5214ED19.40907@FreeBSD.org> <5215047C.8080107@freebsd.org> <521505A3.4020103@FreeBSD.org> <52151368.40803@freebsd.org> <52151799.3020809@FreeBSD.org>

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On 21.08.2013 21:40, Navdeep Parhar wrote:
> On 08/21/13 12:22, Andre Oppermann wrote:
>> On 21.08.2013 20:23, Navdeep Parhar wrote:
>>> I believe we need an extra patch to get M_NOFREE correct.  I've had it
>>> forever in some of my internal repos but never committed it upstream
>>> (just plain forgot).  Since this stuff is fresh in your mind, can you
>>> review this:
>>>
>>> diff -r cd78031b7885 sys/sys/mbuf.h
>>> --- a/sys/sys/mbuf.h    Fri Aug 16 13:35:26 2013 -0700
>>> +++ b/sys/sys/mbuf.h    Wed Aug 21 10:55:57 2013 -0700
>>> @@ -535,6 +535,8 @@ m_free(struct mbuf *m)
>>>    {
>>>        struct mbuf *n = m->m_next;
>>>
>>> +    if ((m->m_flags & (M_PKTHDR|M_NOFREE)) == (M_PKTHDR|M_NOFREE))
>>> +        m_tag_delete_chain(m, NULL);
>>>        if (m->m_flags & M_EXT)
>>>            mb_free_ext(m);
>>>        else if ((m->m_flags & M_NOFREE) == 0)
>>>
>>> It prevents leaks of tags from M_NOFREE+pkthdr mbufs.
>>
>> Good point.  Looks correct.
>>
>> But then I wonder if it is really a smart thing to allow single
>> mbufs (w/o M_EXT) to be M_NOFREE at the same time.  They easily
>> get lost.  If it doesn't have an external buffer attached there
>> is no way to know when and if it was freed.
>>
>> If M_NOFREE is only allowed together with M_EXT then the tag chain
>> delete should happen in mb_ext_free() next to 'skipmbuf' instead.
>>
>> Index: kern/uipc_mbuf.c
>> ===================================================================
>> --- kern/uipc_mbuf.c    (revision 254605)
>> +++ kern/uipc_mbuf.c    (working copy)
>> @@ -283,11 +283,6 @@
>>          KASSERT((m->m_flags & M_EXT) == M_EXT, ("%s: M_EXT not set",
>> __func__));
>>          KASSERT(m->m_ext.ref_cnt != NULL, ("%s: ref_cnt not set",
>> __func__));
>>
>> -       /*
>> -        * check if the header is embedded in the cluster
>> -        */
>> -       skipmbuf = (m->m_flags & M_NOFREE);
>> -
>>          /* Free attached storage if this mbuf is the only reference to
>> it. */
>>          if (*(m->m_ext.ref_cnt) == 1 ||
>>              atomic_fetchadd_int(m->m_ext.ref_cnt, -1) == 1) {
>> @@ -328,8 +323,14 @@
>>                                  ("%s: unknown ext_type", __func__));
>>                  }
>>          }
>> -       if (skipmbuf)
>> +
>> +       /*
>> +        * Do not free if the mbuf is embedded in the cluster.
>> +        */
>> +       if (m->m_flags & M_NOFREE) {
>> +               m_tag_delete_chain(m, NULL);
>>                  return;
>> +       }
>
> The problem with this is that the mbuf may already be gone if it was
> embedded in the cluster that was just freed by the ext free.  That's why
> skipmbuf is used to cache the M_NOFREE bit.

Next try: ;)

Index: kern/uipc_mbuf.c
===================================================================
--- kern/uipc_mbuf.c    (revision 254605)
+++ kern/uipc_mbuf.c    (working copy)
@@ -284,9 +284,12 @@
         KASSERT(m->m_ext.ref_cnt != NULL, ("%s: ref_cnt not set", __func__));

         /*
-        * check if the header is embedded in the cluster
+        * Do not free if the mbuf is embedded in the cluster
+        * but make sure to get rid of any tag chains first.
          */
         skipmbuf = (m->m_flags & M_NOFREE);
+       if (skipmbuf && (m->m_flags & M_PKTHDR))
+               m_tag_delete_chain(m, NULL);

         /* Free attached storage if this mbuf is the only reference to it. */
         if (*(m->m_ext.ref_cnt) == 1 ||

-- 
Andre




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