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The m_ext.ext_cnt pointer becomes a union. It can now hold the refcount value itself. To tell that m_ext.ext_flags flag EXT_FLAG_EMBREF is used. The first mbuf to attach a cluster stores the refcount. The further mbufs to reference the cluster point at refcount in the first mbuf. The first mbuf is freed only when the last reference is freed. The benefit over refcounts stored in separate slabs is that now refcounts of different, unrelated mbufs do not share a cache line. For EXT_EXTREF mbufs the zone_ext_refcnt is no longer needed, and m_extadd() becomes void, making widely used M_EXTADD macro safe. For EXT_SFBUF mbufs the sf_ext_ref() is removed, which was an optimization exactly against the cache aliasing problem with regular refcounting. Discussed with: rrs, rwatson, gnn, hiren, sbruno, np Reviewed by: rrs Differential Revision: https://reviews.freebsd.org/D5396 Sponsored by: Netflix
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