runtime: improve efence

Mark free memory blocks as unused.
On amd64 it allows the process to eat all 128 GB of heap
without killing the machine.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/74070043

src/pkg/runtime/mem_darwin.c

@@ -44,7 +44,7 @@ runtime·SysFree(void *v, uintptr n, uint64 *stat)
 void
 runtime·SysFault(void *v, uintptr n)
 {
-	runtime·mmap(v, n, PROT_NONE, 0, -1, 0);
+	runtime·mmap(v, n, PROT_NONE, MAP_ANON|MAP_PRIVATE|MAP_FIXED, -1, 0);
 }
 
 void*

src/pkg/runtime/mem_dragonfly.c

@@ -48,7 +48,7 @@ runtime·SysFree(void *v, uintptr n, uint64 *stat)
 void
 runtime·SysFault(void *v, uintptr n)
 {
-	runtime·mmap(v, n, PROT_NONE, 0, -1, 0);
+	runtime·mmap(v, n, PROT_NONE, MAP_ANON|MAP_PRIVATE|MAP_FIXED, -1, 0);
 }
 
 void*

src/pkg/runtime/mem_freebsd.c

@@ -48,7 +48,7 @@ runtime·SysFree(void *v, uintptr n, uint64 *stat)
 void
 runtime·SysFault(void *v, uintptr n)
 {
-	runtime·mmap(v, n, PROT_NONE, 0, -1, 0);
+	runtime·mmap(v, n, PROT_NONE, MAP_ANON|MAP_PRIVATE|MAP_FIXED, -1, 0);
 }
 
 void*

src/pkg/runtime/mem_linux.c

@@ -95,7 +95,7 @@ runtime·SysFree(void *v, uintptr n, uint64 *stat)
 void
 runtime·SysFault(void *v, uintptr n)
 {
-	runtime·mmap(v, n, PROT_NONE, 0, -1, 0);
+	runtime·mmap(v, n, PROT_NONE, MAP_ANON|MAP_PRIVATE|MAP_FIXED, -1, 0);
 }
 
 void*

src/pkg/runtime/mem_nacl.c

@@ -56,7 +56,7 @@ runtime·SysFree(void *v, uintptr n, uint64 *stat)
 void
 runtime·SysFault(void *v, uintptr n)
 {
-	runtime·mmap(v, n, PROT_NONE, 0, -1, 0);
+	runtime·mmap(v, n, PROT_NONE, MAP_ANON|MAP_PRIVATE|MAP_FIXED, -1, 0);
 }
 
 void*

src/pkg/runtime/mem_netbsd.c

@@ -48,7 +48,7 @@ runtime·SysFree(void *v, uintptr n, uint64 *stat)
 void
 runtime·SysFault(void *v, uintptr n)
 {
-	runtime·mmap(v, n, PROT_NONE, 0, -1, 0);
+	runtime·mmap(v, n, PROT_NONE, MAP_ANON|MAP_PRIVATE|MAP_FIXED, -1, 0);
 }
 
 void*

src/pkg/runtime/mem_openbsd.c

@@ -48,7 +48,7 @@ runtime·SysFree(void *v, uintptr n, uint64 *stat)
 void
 runtime·SysFault(void *v, uintptr n)
 {
-	runtime·mmap(v, n, PROT_NONE, 0, -1, 0);
+	runtime·mmap(v, n, PROT_NONE, MAP_ANON|MAP_PRIVATE|MAP_FIXED, -1, 0);
 }
 
 void*

src/pkg/runtime/mem_solaris.c

@@ -49,7 +49,7 @@ runtime·SysFree(void *v, uintptr n, uint64 *stat)
 void
 runtime·SysFault(void *v, uintptr n)
 {
-	runtime·mmap(v, n, PROT_NONE, 0, -1, 0);
+	runtime·mmap(v, n, PROT_NONE, MAP_ANON|MAP_PRIVATE|MAP_FIXED, -1, 0);
 }
 
 void*

src/pkg/runtime/mem_windows.c

@@ -66,11 +66,8 @@ runtime·SysFree(void *v, uintptr n, uint64 *stat)
 void
 runtime·SysFault(void *v, uintptr n)
 {
-	uintptr r, old;
-
-	r = (uintptr)runtime·stdcall(runtime·VirtualProtect, 4, v, n, (uintptr)PAGE_NOACCESS, &old);
-	if(r == 0)
-		runtime·throw("runtime: failed to protect pages");
+	// SysUnused makes the memory inaccessible and prevents its reuse
+	runtime·SysUnused(v, n);
 }
 
 void*