From 5a5232ebb88f2a3813d3f747f39a10c6652ae927 Mon Sep 17 00:00:00 2001
From: David du Colombier <0intro@gmail.com>
Date: Wed, 29 May 1991 00:00:00 +0000
Subject: [PATCH] Plan 9 from Bell Labs 1991-05-29

---
 port/proc.c    |  7 ++++++-
 port/sysproc.c | 12 ++++++++++--
 2 files changed, 16 insertions(+), 3 deletions(-)

diff --git a/port/proc.c b/port/proc.c
index f48ba79badcb4795f7bd7430bdb6b60471450250..032ed66b5b34a9c909dbc2a323a7bcda11e738f3 100644
--- a/port/proc.c
+++ b/port/proc.c
@@ -606,7 +606,7 @@ kproc(char *name, void (*func)(void *), void *arg)
 	/*
 	 * Save time: only copy u-> data and useful stack
 	 */
-	clearmmucache();
+	clearmmucache();	/* so child doesn't inherit any of your mappings */
 	memmove(up, u, sizeof(User));
 	n = USERADDR+BY2PG - (ulong)&lastvar;
 	n = (n+32) & ~(BY2WD-1);	/* be safe & word align */
@@ -645,5 +645,10 @@ kproc(char *name, void (*func)(void *), void *arg)
 	memset(p->time, 0, sizeof(p->time));
 	p->time[TReal] = MACHP(0)->ticks;
 	ready(p);
+	/*
+	 *  since the bss/data segments are now shareable,
+	 *  any mmu info about this process is now stale
+	 *  and has to be discarded.
+	 */
 	flushmmu();
 }
diff --git a/port/sysproc.c b/port/sysproc.c
index 0a2535c2b21a81f7acb97c68db108a0aab146fb1..7921d4592e0fc9d33dd8414b4ee539c194838eac 100644
--- a/port/sysproc.c
+++ b/port/sysproc.c
@@ -41,7 +41,7 @@ sysfork(ulong *arg)
 	/*
 	 * Save time: only copy u-> data and useful stack
 	 */
-	clearmmucache();
+	clearmmucache();	/* so child doesn't inherit any of your mappings */
 	memmove((void*)upa, u, sizeof(User));
 	n = USERADDR+BY2PG - (ulong)&lastvar;
 	n = (n+32) & ~(BY2WD-1);	/* be safe & word align */
@@ -115,7 +115,6 @@ sysfork(ulong *arg)
 	 * Sched
 	 */
 	if(setlabel(&p->sched)){
-		clearmmucache();
 		u->p = p;
 		p->state = Running;
 		p->mach = m;
@@ -134,6 +133,11 @@ sysfork(ulong *arg)
 	memset(p->time, 0, sizeof(p->time));
 	p->time[TReal] = MACHP(0)->ticks;
 	memmove(p->text, u->p->text, NAMELEN);
+	/*
+	 *  since the bss/data segments are now shareable,
+	 *  any mmu info about this process is now stale
+	 *  (i.e. has bad properties) and has to be discarded.
+	 */
 	flushmmu();
 	clearmmucache();
 	ready(p);
@@ -351,6 +355,10 @@ sysexec(ulong *arg)
 		o->pte[i].page->va += (USTKTOP-TSTKTOP);
 	unlock(o);
 
+	/*
+	 *  at this point, the mmu contains info about the old address
+	 *  space and needs to be flushed
+	 */
 	flushmmu();
 	clearmmucache();
 	execpc(exec.entry);