Re: [PATCH v1 02/11] mm/pagewalk: introduce folio_walk_start() + folio_walk_end()

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On Fri,  2 Aug 2024 17:55:15 +0200
David Hildenbrand <david@xxxxxxxxxx> wrote:

> We want to get rid of follow_page(), and have a more reasonable way to
> just lookup a folio mapped at a certain address, perform some checks while
> still under PTL, and then only conditionally grab a folio reference if
> really required.
> 
> Further, we might want to get rid of some walk_page_range*() users that
> really only want to temporarily lookup a single folio at a single address.
> 
> So let's add a new page table walker that does exactly that, similarly
> to GUP also being able to walk hugetlb VMAs.
> 
> Add folio_walk_end() as a macro for now: the compiler is not easy to
> please with the pte_unmap()->kunmap_local().
> 
> Note that one difference between follow_page() and get_user_pages(1) is
> that follow_page() will not trigger faults to get something mapped. So
> folio_walk is at least currently not a replacement for get_user_pages(1),
> but could likely be extended/reused to achieve something similar in the
> future.

[...]

> +struct folio *folio_walk_start(struct folio_walk *fw,
> +		struct vm_area_struct *vma, unsigned long addr,
> +		folio_walk_flags_t flags)
> +{
> +	unsigned long entry_size;
> +	bool expose_page = true;
> +	struct page *page;
> +	pud_t *pudp, pud;
> +	pmd_t *pmdp, pmd;
> +	pte_t *ptep, pte;
> +	spinlock_t *ptl;
> +	pgd_t *pgdp;
> +	p4d_t *p4dp;
> +
> +	mmap_assert_locked(vma->vm_mm);
> +	vma_pgtable_walk_begin(vma);
> +
> +	if (WARN_ON_ONCE(addr < vma->vm_start || addr >= vma->vm_end))
> +		goto not_found;
> +
> +	pgdp = pgd_offset(vma->vm_mm, addr);
> +	if (pgd_none_or_clear_bad(pgdp))
> +		goto not_found;
> +
> +	p4dp = p4d_offset(pgdp, addr);
> +	if (p4d_none_or_clear_bad(p4dp))
> +		goto not_found;
> +
> +	pudp = pud_offset(p4dp, addr);
> +	pud = pudp_get(pudp);
> +	if (pud_none(pud))
> +		goto not_found;
> +	if (IS_ENABLED(CONFIG_PGTABLE_HAS_HUGE_LEAVES) && pud_leaf(pud)) {
> +		ptl = pud_lock(vma->vm_mm, pudp);
> +		pud = pudp_get(pudp);
> +
> +		entry_size = PUD_SIZE;
> +		fw->level = FW_LEVEL_PUD;
> +		fw->pudp = pudp;
> +		fw->pud = pud;
> +
> +		if (!pud_present(pud) || pud_devmap(pud)) {
> +			spin_unlock(ptl);
> +			goto not_found;
> +		} else if (!pud_leaf(pud)) {
> +			spin_unlock(ptl);
> +			goto pmd_table;
> +		}
> +		/*
> +		 * TODO: vm_normal_page_pud() will be handy once we want to
> +		 * support PUD mappings in VM_PFNMAP|VM_MIXEDMAP VMAs.
> +		 */
> +		page = pud_page(pud);
> +		goto found;
> +	}
> +
> +pmd_table:
> +	VM_WARN_ON_ONCE(pud_leaf(*pudp));

is this warning necessary? can this actually happen?
and if it can happen, wouldn't it be more reasonable to return NULL?

> +	pmdp = pmd_offset(pudp, addr);
> +	pmd = pmdp_get_lockless(pmdp);
> +	if (pmd_none(pmd))
> +		goto not_found;
> +	if (IS_ENABLED(CONFIG_PGTABLE_HAS_HUGE_LEAVES) && pmd_leaf(pmd)) {
> +		ptl = pmd_lock(vma->vm_mm, pmdp);
> +		pmd = pmdp_get(pmdp);
> +
> +		entry_size = PMD_SIZE;
> +		fw->level = FW_LEVEL_PMD;
> +		fw->pmdp = pmdp;
> +		fw->pmd = pmd;
> +
> +		if (pmd_none(pmd)) {
> +			spin_unlock(ptl);
> +			goto not_found;
> +		} else if (!pmd_leaf(pmd)) {
> +			spin_unlock(ptl);
> +			goto pte_table;
> +		} else if (pmd_present(pmd)) {
> +			page = vm_normal_page_pmd(vma, addr, pmd);
> +			if (page) {
> +				goto found;
> +			} else if ((flags & FW_ZEROPAGE) &&
> +				    is_huge_zero_pmd(pmd)) {
> +				page = pfn_to_page(pmd_pfn(pmd));
> +				expose_page = false;
> +				goto found;
> +			}
> +		} else if ((flags & FW_MIGRATION) &&
> +			   is_pmd_migration_entry(pmd)) {
> +			swp_entry_t entry = pmd_to_swp_entry(pmd);
> +
> +			page = pfn_swap_entry_to_page(entry);
> +			expose_page = false;
> +			goto found;
> +		}
> +		spin_unlock(ptl);
> +		goto not_found;
> +	}
> +
> +pte_table:
> +	VM_WARN_ON_ONCE(pmd_leaf(pmdp_get_lockless(pmdp)));

same here

> +	ptep = pte_offset_map_lock(vma->vm_mm, pmdp, addr, &ptl);
> +	if (!ptep)
> +		goto not_found;
> +	pte = ptep_get(ptep);
> +
> +	entry_size = PAGE_SIZE;
> +	fw->level = FW_LEVEL_PTE;
> +	fw->ptep = ptep;
> +	fw->pte = pte;
> +
> +	if (pte_present(pte)) {
> +		page = vm_normal_page(vma, addr, pte);
> +		if (page)
> +			goto found;
> +		if ((flags & FW_ZEROPAGE) &&
> +		    is_zero_pfn(pte_pfn(pte))) {
> +			page = pfn_to_page(pte_pfn(pte));
> +			expose_page = false;
> +			goto found;
> +		}
> +	} else if (!pte_none(pte)) {
> +		swp_entry_t entry = pte_to_swp_entry(pte);
> +
> +		if ((flags & FW_MIGRATION) &&
> +		    is_migration_entry(entry)) {
> +			page = pfn_swap_entry_to_page(entry);
> +			expose_page = false;
> +			goto found;
> +		}
> +	}
> +	pte_unmap_unlock(ptep, ptl);
> +not_found:
> +	vma_pgtable_walk_end(vma);
> +	return NULL;
> +found:
> +	if (expose_page)
> +		/* Note: Offset from the mapped page, not the folio start. */
> +		fw->page = nth_page(page, (addr & (entry_size - 1)) >> PAGE_SHIFT);
> +	else
> +		fw->page = NULL;
> +	fw->ptl = ptl;
> +	return page_folio(page);
> +}





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