RE: [PATCH 1/1] Implement shared page tables

["Chen, Kenneth W" <kenneth.w.chen@intel.com>]

Dave McCracken wrote on Tuesday, August 30, 2005 3:13 PM
> This patch implements page table sharing for all shared memory regions that
> span an entire page table page.  It supports sharing at multiple page
> levels, depending on the architecture.
> 
> 
> This version of the patch supports i386 and x86_64.  I have additional
> patches to support ppc64, but they are not quite ready for public
> consumption.
> 
>  ....
> +		prio_tree_iter_init(&iter, &mapping->i_mmap,
> +				    vma->vm_start, vma->vm_end);


I think this is a bug.  The radix priority tree for address_space->
i_mmap is keyed on vma->vm_pgoff.  Your patch uses the vma virtual
address to find a shareable range, Which will always fail a match
even though there is one.  The following is a quick hack I did to
make it work.

- Ken

--- linux-2.6.13/mm/ptshare.c.orig	2005-09-01 18:58:12.299321918 -0700
+++ linux-2.6.13/mm/ptshare.c	2005-09-01 18:58:39.846196580 -0700
@@ -26,6 +26,11 @@
 #include <asm/pgtable.h>
 #include <asm/pgalloc.h>
 
+#define RADIX_INDEX(vma)  ((vma)->vm_pgoff)
+#define VMA_SIZE(vma)	  (((vma)->vm_end - (vma)->vm_start) >> PAGE_SHIFT)
+/* avoid overflow */
+#define HEAP_INDEX(vma)	  ((vma)->vm_pgoff + (VMA_SIZE(vma) - 1))
+
 #undef	PT_DEBUG
 
 #ifndef __PAGETABLE_PMD_FOLDED
@@ -173,7 +178,7 @@ pt_share_pte(struct vm_area_struct *vma,
 		       address);
 #endif
 		prio_tree_iter_init(&iter, &mapping->i_mmap,
-				    vma->vm_start, vma->vm_end);
+				    RADIX_INDEX(vma), HEAP_INDEX(vma));
 
 		while ((svma = next_shareable_vma(vma, svma, &iter))) {
 			spgd = pgd_offset(svma->vm_mm, address);


-
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