Module Name: src
Committed By: riastradh
Date: Sat Jun 4 20:54:53 UTC 2022
Modified Files:
src/sys/uvm: uvm_map.c
Log Message:
uvm(9): Fix mmap optimization for topdown case.
PR kern/51393
To generate a diff of this commit:
cvs rdiff -u -r1.395 -r1.396 src/sys/uvm/uvm_map.c
Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.
Modified files:
Index: src/sys/uvm/uvm_map.c
diff -u src/sys/uvm/uvm_map.c:1.395 src/sys/uvm/uvm_map.c:1.396
--- src/sys/uvm/uvm_map.c:1.395 Sat Jun 4 20:54:24 2022
+++ src/sys/uvm/uvm_map.c Sat Jun 4 20:54:53 2022
@@ -1,4 +1,4 @@
-/* $NetBSD: uvm_map.c,v 1.395 2022/06/04 20:54:24 riastradh Exp $ */
+/* $NetBSD: uvm_map.c,v 1.396 2022/06/04 20:54:53 riastradh Exp $ */
/*
* Copyright (c) 1997 Charles D. Cranor and Washington University.
@@ -66,7 +66,7 @@
*/
#include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: uvm_map.c,v 1.395 2022/06/04 20:54:24 riastradh Exp $");
+__KERNEL_RCSID(0, "$NetBSD: uvm_map.c,v 1.396 2022/06/04 20:54:53 riastradh Exp $");
#include "opt_ddb.h"
#include "opt_pax.h"
@@ -1876,7 +1876,36 @@ uvm_map_findspace(struct vm_map *map, va
* it, there would be four cases).
*/
- if ((flags & UVM_FLAG_FIXED) == 0 && hint == vm_map_min(map)) {
+ if ((flags & UVM_FLAG_FIXED) == 0 &&
+ hint == (topdown ? vm_map_max(map) : vm_map_min(map))) {
+ /*
+ * The uvm_map_findspace algorithm is monotonic -- for
+ * topdown VM it starts with a high hint and returns a
+ * lower free address; for !topdown VM it starts with a
+ * low hint and returns a higher free address. As an
+ * optimization, start with the first (highest for
+ * topdown, lowest for !topdown) free address.
+ *
+ * XXX This `optimization' probably doesn't actually do
+ * much in practice unless userland explicitly passes
+ * the VM map's minimum or maximum address, which
+ * varies from machine to machine (VM_MAX/MIN_ADDRESS,
+ * e.g. 0x7fbfdfeff000 on amd64 but 0xfffffffff000 on
+ * aarch64) and may vary according to other factors
+ * like sysctl vm.user_va0_disable. In particular, if
+ * the user specifies 0 as a hint to mmap, then mmap
+ * will choose a default address which is usually _not_
+ * VM_MAX/MIN_ADDRESS but something else instead like
+ * VM_MAX_ADDRESS - stack size - guard page overhead,
+ * in which case this branch is never hit.
+ *
+ * In fact, this branch appears to have been broken for
+ * two decades between when topdown was introduced in
+ * ~2003 and when it was adapted to handle the topdown
+ * case without violating the monotonicity assertion in
+ * 2022. Maybe Someone^TM should either ditch the
+ * optimization or find a better way to do it.
+ */
entry = map->first_free;
} else {
if (uvm_map_lookup_entry(map, hint, &entry)) {
