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Date:      Fri, 10 Jan 2014 14:20:32 +0400
From:      Gleb Smirnoff <glebius@FreeBSD.org>
To:        Kevin Lo <kevlo@FreeBSD.org>
Cc:        svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers@freebsd.org
Subject:   Re: svn commit: r260501 - head/sys/dev/usb/wlan
Message-ID:  <20140110102032.GC73147@FreeBSD.org>
In-Reply-To: <201401100247.s0A2lK9q066888@svn.freebsd.org>
References:  <201401100247.s0A2lK9q066888@svn.freebsd.org>
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 Kevin,

On Fri, Jan 10, 2014 at 02:47:20AM +0000, Kevin Lo wrote:
K> Author: kevlo
K> Date: Fri Jan 10 02:47:20 2014
K> New Revision: 260501
K> URL: http://svnweb.freebsd.org/changeset/base/260501
K> 
K> Log:
K>   Use m_getcl() instead of MGETHDR/MCLGET macros.
K>   
K>   Suggested by:	glebius
K> 
K> Modified:
K>   head/sys/dev/usb/wlan/if_rsu.c
K> 
K> Modified: head/sys/dev/usb/wlan/if_rsu.c
K> ==============================================================================
K> --- head/sys/dev/usb/wlan/if_rsu.c	Fri Jan 10 01:44:34 2014	(r260500)
K> +++ head/sys/dev/usb/wlan/if_rsu.c	Fri Jan 10 02:47:20 2014	(r260501)
K> @@ -1145,16 +1145,9 @@ rsu_event_survey(struct rsu_softc *sc, u
K>  	pktlen = sizeof(*wh) + le32toh(bss->ieslen);
K>  	if (__predict_false(pktlen > MCLBYTES))
K>  		return;
K> -	MGETHDR(m, M_NOWAIT, MT_DATA);
K> +	m = m_getcl(M_NOWAIT, MT_DATA, M_PKTHDR);
K>  	if (__predict_false(m == NULL))
K>  		return;
K> -	if (pktlen > MHLEN) {
K> -		MCLGET(m, M_NOWAIT);
K> -		if (!(m->m_flags & M_EXT)) {
K> -			m_free(m);
K> -			return;
K> -		}
K> -	}
K>  	wh = mtod(m, struct ieee80211_frame *);
K>  	wh->i_fc[0] = IEEE80211_FC0_VERSION_0 | IEEE80211_FC0_TYPE_MGT |
K>  	    IEEE80211_FC0_SUBTYPE_BEACON;
K> @@ -1358,19 +1351,11 @@ rsu_rx_frame(struct rsu_softc *sc, uint8
K>  	DPRINTFN(5, "Rx frame len=%d rate=%d infosz=%d rssi=%d\n",
K>  	    pktlen, rate, infosz, *rssi);
K>  
K> -	MGETHDR(m, M_NOWAIT, MT_DATA);
K> +	m = m_getcl(M_NOWAIT, MT_DATA, M_PKTHDR);
K>  	if (__predict_false(m == NULL)) {
K>  		ifp->if_ierrors++;
K>  		return NULL;
K>  	}
K> -	if (pktlen > MHLEN) {
K> -		MCLGET(m, M_NOWAIT);
K> -		if (__predict_false(!(m->m_flags & M_EXT))) {
K> -			ifp->if_ierrors++;
K> -			m_freem(m);
K> -			return NULL;
K> -		}
K> -	}
K>  	/* Finalize mbuf. */
K>  	m->m_pkthdr.rcvif = ifp;
K>  	/* Hardware does Rx TCP checksum offload. */

Sorry, but the correct code would be:

	if (pktlen > MHLEN)
		m = m_getcl(M_NOWAIT, MT_DATA, M_PKTHDR);
	else
		m = m_gethdr(M_NOWAIT, MT_DATA);
	if (__predict_false(m == NULL)) {
		ifp->if_ierrors++;
		return NULL;
	}

Alternatively, you can use:

	m = m_get2(pktlen, M_NOWAIT, MT_DATA, M_PKTHDR);
	if (__predict_false(m == NULL)) {
		ifp->if_ierrors++;
		return NULL;
	}

With committed code we are wasting memory for small packets.

-- 
Totus tuus, Glebius.

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