Date: Wed, 21 Aug 2013 22:59:16 +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: <52152A24.8030109@freebsd.org> In-Reply-To: <5215288E.6070708@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> <52152697.9070500@freebsd.org> <5215288E.6070708@FreeBSD.org>
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On 21.08.2013 22:52, Navdeep Parhar wrote:
> 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?
I don't mind having your patch. Though I don't see how it possibly can't leak
mbufs if they are not M_EXT.
--
Andre
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