M_NOFREE removal (was Re: svn commit: r254520 - in head/sys: kern sys)

[Navdeep Parhar]

On 08/21/13 13:44, Andre Oppermann wrote:
> 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: ;)

It is most flexible to let M_NOFREE work without any assumptions
attached (must be M_EXT, etc.)  So I still prefer my patch to this.  If
you don't have any strong preferences may I commit that one instead?

Regards,
Navdeep