[src/trunk]: src/sys/uvm uvm(9): Fix mmap optimization for topdown case.

[riastradh <riastradh%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/a8fbd33d984b
branches:  trunk
changeset: 366650:a8fbd33d984b
user:      riastradh <riastradh%NetBSD.org@localhost>
date:      Sat Jun 04 20:54:53 2022 +0000

description:
uvm(9): Fix mmap optimization for topdown case.

PR kern/51393

diffstat:

 sys/uvm/uvm_map.c |  35 ++++++++++++++++++++++++++++++++---
 1 files changed, 32 insertions(+), 3 deletions(-)

diffs (56 lines):

diff -r 8a5a93637c79 -r a8fbd33d984b sys/uvm/uvm_map.c
--- a/sys/uvm/uvm_map.c Sat Jun 04 20:54:24 2022 +0000
+++ b/sys/uvm/uvm_map.c Sat Jun 04 20:54:53 2022 +0000
@@ -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 @@
         * 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)) {