Re: [PATCH 1/8] mm: support madvise(MADV_FREE)

[Shaohua Li]

On Fri, Oct 30, 2015 at 04:01:37PM +0900, Minchan Kim wrote:
> +static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
> +				unsigned long end, struct mm_walk *walk)
> +
> +{
> +	struct mmu_gather *tlb = walk->private;
> +	struct mm_struct *mm = tlb->mm;
> +	struct vm_area_struct *vma = walk->vma;
> +	spinlock_t *ptl;
> +	pte_t *pte, ptent;
> +	struct page *page;
> +
> +	split_huge_page_pmd(vma, addr, pmd);
> +	if (pmd_trans_unstable(pmd))
> +		return 0;
> +
> +	pte = pte_offset_map_lock(mm, pmd, addr, &ptl);
> +	arch_enter_lazy_mmu_mode();
> +	for (; addr != end; pte++, addr += PAGE_SIZE) {
> +		ptent = *pte;
> +
> +		if (!pte_present(ptent))
> +			continue;
> +
> +		page = vm_normal_page(vma, addr, ptent);
> +		if (!page)
> +			continue;
> +
> +		if (PageSwapCache(page)) {
> +			if (!trylock_page(page))
> +				continue;
> +
> +			if (!try_to_free_swap(page)) {
> +				unlock_page(page);
> +				continue;
> +			}
> +
> +			ClearPageDirty(page);
> +			unlock_page(page);
> +		}
> +
> +		/*
> +		 * Some of architecture(ex, PPC) don't update TLB
> +		 * with set_pte_at and tlb_remove_tlb_entry so for
> +		 * the portability, remap the pte with old|clean
> +		 * after pte clearing.
> +		 */
> +		ptent = ptep_get_and_clear_full(mm, addr, pte,
> +						tlb->fullmm);
> +		ptent = pte_mkold(ptent);
> +		ptent = pte_mkclean(ptent);
> +		set_pte_at(mm, addr, pte, ptent);
> +		tlb_remove_tlb_entry(tlb, pte, addr);

The orginal ptent might not be dirty. In that case, the tlb_remove_tlb_entry
is unnecessary, so please add a check. In practice, I saw more TLB flush with
FREE compared to DONTNEED because of this issue.

Thanks,
Shaohua
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