Re: [PATCHv4 08/25] thp: support file pages in zap_huge_pmd()

[Aneesh Kumar K.V]

"Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx> writes:

> [ text/plain ]
> split_huge_pmd() for file mappings (and DAX too) is implemented by just
> clearing pmd entry as we can re-fill this area from page cache on pte
> level later.
>
> This means we don't need deposit page tables when file THP is mapped.
> Therefore we shouldn't try to withdraw a page table on zap_huge_pmd()
> file THP PMD.

Archs like ppc64 use deposited page table to track the hardware page
table slot information. We probably may want to add hooks which arch can
use to achieve the same even with file THP 

>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> ---
>  mm/huge_memory.c | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 44468fb7cdbf..c22144e3fe11 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1684,10 +1684,16 @@ int zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
>  		struct page *page = pmd_page(orig_pmd);
>  		page_remove_rmap(page, true);
>  		VM_BUG_ON_PAGE(page_mapcount(page) < 0, page);
> -		add_mm_counter(tlb->mm, MM_ANONPAGES, -HPAGE_PMD_NR);
>  		VM_BUG_ON_PAGE(!PageHead(page), page);
> -		pte_free(tlb->mm, pgtable_trans_huge_withdraw(tlb->mm, pmd));
> -		atomic_long_dec(&tlb->mm->nr_ptes);
> +		if (PageAnon(page)) {
> +			pgtable_t pgtable;
> +			pgtable = pgtable_trans_huge_withdraw(tlb->mm, pmd);
> +			pte_free(tlb->mm, pgtable);
> +			atomic_long_dec(&tlb->mm->nr_ptes);
> +			add_mm_counter(tlb->mm, MM_ANONPAGES, -HPAGE_PMD_NR);
> +		} else {
> +			add_mm_counter(tlb->mm, MM_FILEPAGES, -HPAGE_PMD_NR);
> +		}
>  		spin_unlock(ptl);
>  		tlb_remove_page(tlb, page);
>  	}
> -- 
> 2.7.0
>
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