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

Andre Oppermann andre at freebsd.org
Wed Aug 21 19:22:28 UTC 2013


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;
+       }

         /*
          * Free this mbuf back to the mbuf zone with all m_ext

-- 
Andre



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