Re: [PATCH v5 07/11] thp: implement splitting pmd for huge zero page

David Rientjes
2012-11-14T23:28:18Z

On Wed, 7 Nov 2012, Kirill A. Shutemov wrote:

> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 90e651c..f36bc7d 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1611,6 +1611,7 @@ int split_huge_page(struct page *page)
> struct anon_vma *anon_vma;
> int ret = 1;
>
> + BUG_ON(is_huge_zero_pfn(page_to_pfn(page)));
> BUG_ON(!PageAnon(page));
> anon_vma = page_lock_anon_vma(page);
> if (!anon_vma)
> @@ -2509,23 +2510,63 @@ static int khugepaged(void *none)
> return 0;
> }
>
> +static void __split_huge_zero_page_pmd(struct vm_area_struct *vma,
> + unsigned long haddr, pmd_t *pmd)
> +{

This entire function duplicates other code in mm/huge_memory.c which gives
even more incentive into breaking do_huge_pmd_wp_zero_page_fallback() into
logical helper functions and reusing them for both page and !page.
Duplicating all this code throughout the thp code just becomes a
maintenance nightmare down the road.

> + pgtable_t pgtable;
> + pmd_t _pmd;
> + int i;
> +
> + pmdp_clear_flush(vma, haddr, pmd);
> + /* leave pmd empty until pte is filled */
> +
> + pgtable = get_pmd_huge_pte(vma->vm_mm);
> + pmd_populate(vma->vm_mm, &_pmd, pgtable);
> +
> + for (i = 0; i < HPAGE_PMD_NR; i++, haddr += PAGE_SIZE) {
> + pte_t *pte, entry;
> + entry = pfn_pte(my_zero_pfn(haddr), vma->vm_page_prot);
> + entry = pte_mkspecial(entry);
> + pte = pte_offset_map(&_pmd, haddr);
> + VM_BUG_ON(!pte_none(*pte));
> + set_pte_at(vma->vm_mm, haddr, pte, entry);
> + pte_unmap(pte);
> + }
> + smp_wmb(); /* make pte visible before pmd */
> + pmd_populate(vma->vm_mm, pmd, pgtable);
> +}
> +
> void __split_huge_page_pmd(struct vm_area_struct *vma, unsigned long address,
> pmd_t *pmd)
> {
> struct page *page;
> + struct mm_struct *mm = vma->vm_mm;
> unsigned long haddr = address & HPAGE_PMD_MASK;
> + unsigned long mmun_start; /* For mmu_notifiers */
> + unsigned long mmun_end; /* For mmu_notifiers */
>
> BUG_ON(vma->vm_start > haddr || vma->vm_end < haddr + HPAGE_PMD_SIZE);
>
> - spin_lock(&vma->vm_mm->page_table_lock);
> + mmun_start = haddr;
> + mmun_end = address + HPAGE_PMD_SIZE;

address or haddr?

> + mmu_notifier_invalidate_range_start(mm, mmun_start, mmun_end);
> + spin_lock(&mm->page_table_lock);
> if (unlikely(!pmd_trans_huge(*pmd))) {
> - spin_unlock(&vma->vm_mm->page_table_lock);
> + spin_unlock(&mm->page_table_lock);
> + mmu_notifier_invalidate_range_end(mm, mmun_start, mmun_end);
> + return;
> + }
> + if (is_huge_zero_pmd(*pmd)) {
> + __split_huge_zero_page_pmd(vma, haddr, pmd);
> + spin_unlock(&mm->page_table_lock);
> + mmu_notifier_invalidate_range_end(mm, mmun_start, mmun_end);
> return;
> }
> page = pmd_page(*pmd);
> VM_BUG_ON(!page_count(page));
> get_page(page);
> - spin_unlock(&vma->vm_mm->page_table_lock);
> + spin_unlock(&mm->page_table_lock);
> + mmu_notifier_invalidate_range_end(mm, mmun_start, mmun_end);
>
> split_huge_page(page);
>