svn commit: r196318 - head/sys/amd64/amd64

Konstantin Belousov kib at
Mon Aug 17 13:27:56 UTC 2009

Author: kib
Date: Mon Aug 17 13:27:55 2009
New Revision: 196318

  Correct a critical accounting error in pmap_demote_pde().  Specifically,
  when pmap_demote_pde() allocates a page table page to implement a
  user-space demotion, it must increment the pmap's resident page count.
  Not doing so, can lead to an underflow during address space termination
  that causes pmap_remove() to exit prematurely, before it has destroyed
  all of the mappings within the specified range.  The ultimate effect or
  symptom of this error is an assertion failure in vm_page_free_toq()
  because the page being freed is still mapped.
  This error is only possible when superpage promotion is enabled.  Thus,
  it only affects FreeBSD  versions greater than 7.2.
  Tested by:	pho, alc
  Reviewed by:	alc
  Approved by:	re (rwatson)
  MFC after:	1 week


Modified: head/sys/amd64/amd64/pmap.c
--- head/sys/amd64/amd64/pmap.c	Mon Aug 17 13:00:32 2009	(r196317)
+++ head/sys/amd64/amd64/pmap.c	Mon Aug 17 13:27:55 2009	(r196318)
@@ -2261,6 +2261,8 @@ pmap_demote_pde(pmap_t pmap, pd_entry_t 
 			    " in pmap %p", va, pmap);
 			return (FALSE);
+			pmap->pm_stats.resident_count++;
 	mptepa = VM_PAGE_TO_PHYS(mpte);
 	firstpte = (pt_entry_t *)PHYS_TO_DMAP(mptepa);

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