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From: Konstantin Belousov <kib@FreeBSD.org>
Date: Wed, 15 Apr 2015 06:59:53 +0000 (UTC)
To: src-committers@freebsd.org, svn-src-all@freebsd.org,
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Subject: svn commit: r281547 - stable/10/sys/i386/include
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Author: kib
Date: Wed Apr 15 06:59:53 2015
New Revision: 281547
URL: https://svnweb.freebsd.org/changeset/base/281547

Log:
  MFC r281272:
  Explain that vm_page_array is mapped to describe the memory, not the
  memory itself.  Provide the formula to calculate the number of
  required page tables.  Correct the size of the struct vm_page for
  non-PAE case.

Modified:
  stable/10/sys/i386/include/pmap.h
Directory Properties:
  stable/10/   (props changed)

Modified: stable/10/sys/i386/include/pmap.h
==============================================================================
--- stable/10/sys/i386/include/pmap.h	Wed Apr 15 06:57:47 2015	(r281546)
+++ stable/10/sys/i386/include/pmap.h	Wed Apr 15 06:59:53 2015	(r281547)
@@ -122,13 +122,18 @@
  */
 #define VADDR(pdi, pti) ((vm_offset_t)(((pdi)<<PDRSHIFT)|((pti)<<PAGE_SHIFT)))
 
-/* Initial number of kernel page tables. */
+/*
+ * The initial number of kernel page table pages that are constructed
+ * by locore must be sufficient to map vm_page_array.  That number can
+ * be calculated as follows:
+ *     max_phys / PAGE_SIZE * sizeof(struct vm_page) / NBPDR
+ * PAE:      max_phys 16G, sizeof(vm_page) 76, NBPDR 2M, 152 page table pages.
+ * Non-PAE:  max_phys 4G,  sizeof(vm_page) 68, NBPDR 4M, 18 page table pages.
+ */
 #ifndef NKPT
 #ifdef PAE
-/* 152 page tables needed to map 16G (76B "struct vm_page", 2M page tables). */
 #define	NKPT		240
 #else
-/* 18 page tables needed to map 4G (72B "struct vm_page", 4M page tables). */
 #define	NKPT		30
 #endif
 #endif