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SubjectRe: [PATCH][mm] adjust the logic of checking THP
On Wed, Nov 02, 2011 at 02:34:30PM +0800, Guanjun He wrote:
>
> Acturally, pmd_trans_huge(orig_pmd) only checks the _PAGE_PSE bits,
> it's a pmd entry bits, only mark a size, not a flag;As one can easily
> create the same pmd entry bits for some special use,then the check
> will get confused.And this patch is to adjust the logic to use the flag,
> it can perfectly avoid this potential issuse,and basically no impact
> to the current code.

You can't use _PAGE_PSE for special use for the pmd. Besides this is
common code, archs without such bit can define pmd_trans_huge to
return 0 like it happens with TRANSPARENT_HUGEPAGE=n.

> diff --git a/mm/memory.c b/mm/memory.c
> index a56e3ba..a76b17f 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3465,20 +3465,22 @@ int handle_mm_fault(struct mm_struct *mm, struct vm_area_struct *vma,
> pmd = pmd_alloc(mm, pud, address);
> if (!pmd)
> return VM_FAULT_OOM;
> - if (pmd_none(*pmd) && transparent_hugepage_enabled(vma)) {
> - if (!vma->vm_ops)
> - return do_huge_pmd_anonymous_page(mm, vma, address,
> - pmd, flags);
> - } else {
> - pmd_t orig_pmd = *pmd;
> - barrier();
> - if (pmd_trans_huge(orig_pmd)) {
> - if (flags & FAULT_FLAG_WRITE &&
> - !pmd_write(orig_pmd) &&
> - !pmd_trans_splitting(orig_pmd))
> - return do_huge_pmd_wp_page(mm, vma, address,
> - pmd, orig_pmd);
> + if (transparent_hugepage_enabled(vma)) {
> + if (pmd_none(*pmd)) {
> + if (!vma->vm_ops)
> + return do_huge_pmd_anonymous_page(mm, vma, address,
> + pmd, flags);
> + } else {
> + pmd_t orig_pmd = *pmd;
> + barrier();
> + if (pmd_trans_huge(orig_pmd)) {
> + if (flags & FAULT_FLAG_WRITE &&
> + !pmd_write(orig_pmd) &&
> + !pmd_trans_splitting(orig_pmd))
> + return do_huge_pmd_wp_page(mm, vma, address,
> + pmd, orig_pmd);
> return 0;
> + }

This will infinite loop if you disable THP at runtime while some
mapping needing cow is established.


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