[src/trunk]: src/sys/arch/alpha After a comment by joerg@, go back to using a...

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

details:   https://anonhg.NetBSD.org/src/rev/50d5372274d7
branches:  trunk
changeset: 1021469:50d5372274d7
user:      thorpej <thorpej%NetBSD.org@localhost>
date:      Mon May 31 17:16:04 2021 +0000

description:
After a comment by joerg@, go back to using a dedicated field for the
PT page reference count, but add an XXX comment stating the desire to
find a safely-unused field in the vm_page structure when pages are in-
use as PT pages, so that we can save the 8 bytes per page needed for
this.

diffstat:

 sys/arch/alpha/alpha/pmap.c   |  19 +++++--------------
 sys/arch/alpha/include/pmap.h |  21 +++++++++++++++++++--
 2 files changed, 24 insertions(+), 16 deletions(-)

diffs (91 lines):

diff -r 3d15914a25c4 -r 50d5372274d7 sys/arch/alpha/alpha/pmap.c
--- a/sys/arch/alpha/alpha/pmap.c       Mon May 31 14:38:55 2021 +0000
+++ b/sys/arch/alpha/alpha/pmap.c       Mon May 31 17:16:04 2021 +0000
@@ -1,4 +1,4 @@
-/* $NetBSD: pmap.c,v 1.292 2021/05/30 19:50:23 thorpej Exp $ */
+/* $NetBSD: pmap.c,v 1.293 2021/05/31 17:16:04 thorpej Exp $ */
 
 /*-
  * Copyright (c) 1998, 1999, 2000, 2001, 2007, 2008, 2020
@@ -135,7 +135,7 @@
 
 #include <sys/cdefs.h>                 /* RCS ID & Copyright macro defns */
 
-__KERNEL_RCSID(0, "$NetBSD: pmap.c,v 1.292 2021/05/30 19:50:23 thorpej Exp $");
+__KERNEL_RCSID(0, "$NetBSD: pmap.c,v 1.293 2021/05/31 17:16:04 thorpej Exp $");
 
 #include <sys/param.h>
 #include <sys/systm.h>
@@ -507,8 +507,8 @@
 
        while ((pg = LIST_FIRST(list)) != NULL) {
                LIST_REMOVE(pg, pageq.list);
-               /* Zap any fields we used internally. */
-               atomic_store_relaxed(&pg->loan_count, 0);
+               /* Fix up ref count; it's not always 0 when we get here. */
+               PHYSPAGE_REFCNT_SET(pg, 0);
                uvm_pagefree(pg);
        }
 }
@@ -1893,7 +1893,7 @@
 
        /* Fix up the reference count on the lev1map page. */
        pg = PHYS_TO_VM_PAGE(ALPHA_K0SEG_TO_PHYS((vaddr_t)lev1map));
-       atomic_store_relaxed(&pg->loan_count, 0);
+       PHYSPAGE_REFCNT_SET(pg, 0);
 
        /* Step 3 */
        while ((pv = LIST_FIRST(&pmap->pm_pvents)) != NULL) {
@@ -3470,15 +3470,6 @@
 /******************** misc. functions ********************/
 
 /*
- * Pages that are in-use as page table pages should never be part
- * of a UVM loan, so we'll use that field for our PT page reference
- * count.
- */
-#define        PHYSPAGE_REFCNT(pg)     atomic_load_relaxed(&(pg)->loan_count)
-#define        PHYSPAGE_REFCNT_INC(pg) atomic_inc_uint_nv(&(pg)->loan_count)
-#define        PHYSPAGE_REFCNT_DEC(pg) atomic_dec_uint_nv(&(pg)->loan_count)
-
-/*
  * pmap_physpage_alloc:
  *
  *     Allocate a single page from the VM system and return the
diff -r 3d15914a25c4 -r 50d5372274d7 sys/arch/alpha/include/pmap.h
--- a/sys/arch/alpha/include/pmap.h     Mon May 31 14:38:55 2021 +0000
+++ b/sys/arch/alpha/include/pmap.h     Mon May 31 17:16:04 2021 +0000
@@ -1,4 +1,4 @@
-/* $NetBSD: pmap.h,v 1.96 2021/05/30 19:41:59 thorpej Exp $ */
+/* $NetBSD: pmap.h,v 1.97 2021/05/31 17:16:05 thorpej Exp $ */
 
 /*-
  * Copyright (c) 1998, 1999, 2000, 2001, 2007 The NetBSD Foundation, Inc.
@@ -355,9 +355,26 @@
  */
 #define        __HAVE_VM_PAGE_MD
 struct vm_page_md {
-       uintptr_t pvh_listx;                    /* pv_entry list + attrs */
+       uintptr_t pvh_listx;            /* pv_entry list + attrs */
+       /*
+        * XXX These fields are only needed for pages that are used
+        * as PT pages.  It would be nice to find safely-unused fields
+        * in the vm_page structure that could be used instead.
+        */
+       unsigned int pvh_physpgrefs;    /* # refs as a PT page */
+       unsigned int pvh_spare0;        /* XXX spare field */
 };
 
+/* Reference counting for page table pages. */
+#define        PHYSPAGE_REFCNT(pg)                                             \
+       atomic_load_relaxed(&(pg)->mdpage.pvh_physpgrefs)
+#define        PHYSPAGE_REFCNT_SET(pg, v)                                      \
+       atomic_store_relaxed(&(pg)->mdpage.pvh_physpgrefs, (v))
+#define        PHYSPAGE_REFCNT_INC(pg)                                         \
+       atomic_inc_uint_nv(&(pg)->mdpage.pvh_physpgrefs)
+#define        PHYSPAGE_REFCNT_DEC(pg)                                         \
+       atomic_dec_uint_nv(&(pg)->mdpage.pvh_physpgrefs)
+
 #define        VM_MDPAGE_PVS(pg)                                               \
        ((struct pv_entry *)((pg)->mdpage.pvh_listx & ~3UL))