Date: Wed, 21 Aug 2013 13:52:30 -0700 From: Navdeep Parhar <np@FreeBSD.org> To: Andre Oppermann <andre@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: <5215288E.6070708@FreeBSD.org> In-Reply-To: <52152697.9070500@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>
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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
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