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SubjectRE: IA64 non-contiguous memory space bugs
Date
David Gibson wrote on Tuesday, February 21, 2006 4:14 PM
> Second problem is in the hugepage logic in free_pgtables()
> (mm/memory.c). As far as I can tell it's complete crap, and only
> works by accident, for different accidental reasons on ppc64 and ia64,
> the only archs that have a non-trivial is_hugepage_only_range().
> Except that I'm not sure it does entirely work by accident on ia64:
> suppose a process has a hugepage mapping that begins some way after
> the beginning of the hugepage address range. Before
> hugetlb_free_pgd_range() gets called on that area, it will be called
> on the next normal page VMA down - but with an end address at the
> beginning of the hugepage VMA and so extending into the hugepage
> address range. I don't really understand the ia64 pagetable mapping
> stuff well enough to tell if that's dangerous or not.

Chen, Kenneth W wrote on Tuesday, February 21, 2006 5:32 PM
> I don't see any problem in the ia64 code. The start and end address is
> what the vma specified. Floor and ceiling is just a hint for free_pgtables()
> to free any left over page tables between vma holes (to prev and next).
> As far as I can tell, the code looks fine.


free_pgtables() has partial crap that the check of is_hugepage_only_range()
should be done on the entire vma range, not just the first hugetlb page.
Though, it's not possible to have a hugetlb vma while having normal page
instantiated inside that vma. So the bug is mostly phantom. For
correctness, it should be fixed.


--- linux-2.6.16-rc4/mm/memory.c.orig 2006-02-21 18:33:32.427186571 -0800
+++ linux-2.6.16-rc4/mm/memory.c 2006-02-21 18:37:22.414488441 -0800
@@ -270,6 +270,7 @@ void free_pgtables(struct mmu_gather **t
while (vma) {
struct vm_area_struct *next = vma->vm_next;
unsigned long addr = vma->vm_start;
+ unsigned long end = vma->vm_end;

/*
* Hide vma from rmap and vmtruncate before freeing pgtables
@@ -277,8 +278,8 @@ void free_pgtables(struct mmu_gather **t
anon_vma_unlink(vma);
unlink_file_vma(vma);

- if (is_hugepage_only_range(vma->vm_mm, addr, HPAGE_SIZE)) {
- hugetlb_free_pgd_range(tlb, addr, vma->vm_end,
+ if (is_hugepage_only_range(vma->vm_mm, addr, end - addr)) {
+ hugetlb_free_pgd_range(tlb, addr, end
floor, next? next->vm_start: ceiling);
} else {
/*



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