The MGET section of mbuf(9) asserts that M_DONTWAIT shouldn't be confused with M_NOWAIT ("they are not the same"). In sys/mbuf.h, they are in fact the same, as M_DONTWAIT is merely defined as M_NOWAIT.
Also, M_TRYWAIT is said to wait for kern.ipc.mbuf_wait ticks, a tunable which apparently no longer exists (and really M_TRYWAIT is just M_WAITOK).
I'm not completely sure on the first point, I think the missing
context not copied from mbuf(9) does point out why they are
different. Compare mbuf.9 to sys/mbuf.h and double check.
For the second point, I sent this patch to another developer for
RCS file: /home/ncvs/src/share/man/man9/mbuf.9,v
retrieving revision 1.65
diff -u -u -r1.65 mbuf.9
--- mbuf.9 26 Feb 2007 15:17:19 -0000 1.65
+++ mbuf.9 22 Jan 2008 11:52:59 -0000
@@ -24,7 +24,7 @@
.\" $FreeBSD: src/share/man/man9/mbuf.9,v 1.65 2007/02/26 15:17:19 bms Exp $
-.Dd February 26, 2007
+.Dd January 22, 2008
.Dt MBUF 9
@@ -363,12 +363,8 @@
is set to
.Dv M_TRYWAIT ,
-a failed allocation will result in the caller being put
-to sleep for a designated
-.Xr ( sysctl 8
-number of ticks.
+a failed allocation will result in the caller blocking until
+resources are available and thus never return NULL.
A number of other functions and macros related to
have the same argument because they may
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