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From: Konstantin Belousov <kib@FreeBSD.org>
Date: Wed, 8 Apr 2015 19:46:13 +0000 (UTC)
To: src-committers@freebsd.org, svn-src-all@freebsd.org,
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Subject: svn commit: r281272 - head/sys/i386/include
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Author: kib
Date: Wed Apr  8 19:46:13 2015
New Revision: 281272
URL: https://svnweb.freebsd.org/changeset/base/281272

Log:
  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.
  
  Reviewed by:	alc, jhb (previous version)
  Sponsored by:	The FreeBSD Foundation
  MFC after:	1 week

Modified:
  head/sys/i386/include/pmap.h

Modified: head/sys/i386/include/pmap.h
==============================================================================
--- head/sys/i386/include/pmap.h	Wed Apr  8 19:12:38 2015	(r281271)
+++ head/sys/i386/include/pmap.h	Wed Apr  8 19:46:13 2015	(r281272)
@@ -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