In order to support ongoing work to implement variable-size mbufs, and
more generally make it easier to extend 'struct mbuf in the future', make a number of changes to the data structure: - As we anticipate embedding mbufs headers within variable-size regions of memory in the future, change the definitions of byte arrays embedded in mbufs to be of size [0] rather than [MLEN] and [MHLEN]. In fact, the cxgbe driver already uses 'struct mbuf' on the front of other storage sizes, but we would like the global mbuf allocator do be able to do this as well. - Fold 'struct m_hdr' into 'struct mbuf' itself, eliminating a set of macros that aliased 'mh_foo' field names to 'm_foo' names such as 'm_next'. These present a particular problem as we would like to add new mbuf-header fields -- e.g., 'm_size' -- that, if similarly named via macros, would introduce collisions with many other variable names in the kernel. - Rename 'struct m_ext' to 'struct struct_m_ext' so that we can add compile-time assertions without bumping into the still-extant 'm_ext' macro. - Remove the MSIZE compile-time assertion for 'struct mbuf', but add new assertions for alignment of embedded data arrays (64-bit alignment even on 32-bit platforms), and for the sizes the mbuf header, packet header, and m_ext structure. - Document that these assertions exist in comments in mbuf.h. This change is not intended to cause (non-trivial) behavioural differences, but is a precursor to further mbuf-allocator work. Differential Revision: https://reviews.freebsd.org/D1483 Reviewed by: bz, gnn, np, glebius ("go ahead, I trust you") Sponsored by: EMC / Isilon Storage Division
This commit is contained in:
Robert Watson
parent
a2b8f1cbb7
commit
3d1a9ed34e
Notes:
svn2git
2020-12-20 02:59:44 +00:00
svn path=/head/; revision=277203
@ -88,10 +88,37 @@ SYSCTL_INT(_kern_ipc, OID_AUTO, m_defragrandomfailures, CTLFLAG_RW,
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* Ensure the correct size of various mbuf parameters. It could be off due
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* to compiler-induced padding and alignment artifacts.
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*/
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CTASSERT(sizeof(struct mbuf) == MSIZE);
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CTASSERT(MSIZE - offsetof(struct mbuf, m_dat) == MLEN);
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CTASSERT(MSIZE - offsetof(struct mbuf, m_pktdat) == MHLEN);
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/*
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* mbuf data storage should be 64-bit aligned regardless of architectural
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* pointer size; check this is the case with and without a packet header.
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*/
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CTASSERT(offsetof(struct mbuf, m_dat) % 8 == 0);
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CTASSERT(offsetof(struct mbuf, m_pktdat) % 8 == 0);
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/*
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* While the specific values here don't matter too much (i.e., +/- a few
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* words), we do want to ensure that changes to these values are carefully
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* reasoned about and properly documented. This is especially the case as
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* network-protocol and device-driver modules encode these layouts, and must
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* be recompiled if the structures change. Check these values at compile time
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* against the ones documented in comments in mbuf.h.
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*
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* NB: Possibly they should be documented there via #define's and not just
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* comments.
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*/
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#if defined(__LP64__)
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CTASSERT(offsetof(struct mbuf, m_dat) == 32);
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CTASSERT(sizeof(struct pkthdr) == 56);
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CTASSERT(sizeof(struct struct_m_ext) == 48);
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#else
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CTASSERT(offsetof(struct mbuf, m_dat) == 24);
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CTASSERT(sizeof(struct pkthdr) == 48);
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CTASSERT(sizeof(struct struct_m_ext) == 28);
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#endif
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/*
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* m_get2() allocates minimum mbuf that would fit "size" argument.
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*/
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@ -60,9 +60,15 @@
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* MLEN is data length in a normal mbuf.
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* MHLEN is data length in an mbuf with pktheader.
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* MINCLSIZE is a smallest amount of data that should be put into cluster.
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*
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* Compile-time assertions in uipc_mbuf.c test these values to ensure that
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* they are sensible.
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*/
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#define MLEN ((int)(MSIZE - sizeof(struct m_hdr)))
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#define MHLEN ((int)(MLEN - sizeof(struct pkthdr)))
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struct mbuf;
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#define MHSIZE offsetof(struct mbuf, M_dat.M_databuf)
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#define MPKTHSIZE offsetof(struct mbuf, M_dat.MH.MH_dat.MH_databuf)
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#define MLEN ((int)(MSIZE - MHSIZE))
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#define MHLEN ((int)(MSIZE - MPKTHSIZE))
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#define MINCLSIZE (MHLEN + 1)
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#ifdef _KERNEL
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@ -86,23 +92,6 @@ struct mb_args {
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};
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#endif /* _KERNEL */
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/*
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* Header present at the beginning of every mbuf.
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* Size ILP32: 24
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* LP64: 32
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*/
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struct m_hdr {
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struct mbuf *mh_next; /* next buffer in chain */
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struct mbuf *mh_nextpkt; /* next chain in queue/record */
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caddr_t mh_data; /* location of data */
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int32_t mh_len; /* amount of data in this mbuf */
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uint32_t mh_type:8, /* type of data in this mbuf */
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mh_flags:24; /* flags; see below */
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#if !defined(__LP64__)
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uint32_t mh_pad; /* pad for 64bit alignment */
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#endif
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};
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/*
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* Packet tag structure (see below for details).
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*/
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@ -118,6 +107,8 @@ struct m_tag {
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* Record/packet header in first mbuf of chain; valid only if M_PKTHDR is set.
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* Size ILP32: 48
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* LP64: 56
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* Compile-time assertions in uipc_mbuf.c test these values to ensure that
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* they are correct.
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*/
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struct pkthdr {
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struct ifnet *rcvif; /* rcv interface */
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@ -166,8 +157,10 @@ struct pkthdr {
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* set.
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* Size ILP32: 28
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* LP64: 48
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* Compile-time assertions in uipc_mbuf.c test these values to ensure that
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* they are correct.
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*/
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struct m_ext {
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struct struct_m_ext {
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volatile u_int *ext_cnt; /* pointer to ref count info */
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caddr_t ext_buf; /* start of buffer */
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uint32_t ext_size; /* size of buffer, for ext_free */
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@ -184,24 +177,41 @@ struct m_ext {
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* purposes.
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*/
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struct mbuf {
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struct m_hdr m_hdr;
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/*
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* Header present at the beginning of every mbuf.
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* Size ILP32: 24
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* LP64: 32
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* Compile-time assertions in uipc_mbuf.c test these values to ensure
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* that they are correct.
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*/
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struct mbuf *m_next; /* next buffer in chain */
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struct mbuf *m_nextpkt; /* next chain in queue/record */
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caddr_t m_data; /* location of data */
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int32_t m_len; /* amount of data in this mbuf */
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uint32_t m_type:8, /* type of data in this mbuf */
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m_flags:24; /* flags; see below */
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#if !defined(__LP64__)
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uint32_t m_pad; /* pad for 64bit alignment */
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#endif
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/*
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* A set of optional headers (packet header, external storage header)
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* and internal data storage. Historically, these arrays were sized
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* to MHLEN (space left after a packet header) and MLEN (space left
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* after only a regular mbuf header); they are now variable size in
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* order to support future work on variable-size mbufs.
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*/
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union {
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struct {
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struct pkthdr MH_pkthdr; /* M_PKTHDR set */
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union {
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struct m_ext MH_ext; /* M_EXT set */
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char MH_databuf[MHLEN];
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struct struct_m_ext MH_ext; /* M_EXT set */
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char MH_databuf[0];
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} MH_dat;
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} MH;
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char M_databuf[MLEN]; /* !M_PKTHDR, !M_EXT */
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char M_databuf[0]; /* !M_PKTHDR, !M_EXT */
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} M_dat;
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};
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#define m_next m_hdr.mh_next
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#define m_len m_hdr.mh_len
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#define m_data m_hdr.mh_data
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#define m_type m_hdr.mh_type
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#define m_flags m_hdr.mh_flags
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#define m_nextpkt m_hdr.mh_nextpkt
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#define m_pkthdr M_dat.MH.MH_pkthdr
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#define m_ext M_dat.MH.MH_dat.MH_ext
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#define m_pktdat M_dat.MH.MH_dat.MH_databuf
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