Re: [PATCH 52/75] mm/rmap: Convert try_to_migrate() to folios

[Zi Yan]

On 4 Feb 2022, at 14:58, Matthew Wilcox (Oracle) wrote:

> Convert the callers to pass a folio and the try_to_migrate_one()
> worker to use a folio throughout.  Fixes an assumption that a
> folio must be <= PMD size.
>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> ---
>  include/linux/rmap.h |  2 +-
>  mm/huge_memory.c     |  4 ++--
>  mm/migrate.c         | 12 ++++++----
>  mm/rmap.c            | 57 +++++++++++++++++++++++---------------------
>  4 files changed, 41 insertions(+), 34 deletions(-)
>
> diff --git a/include/linux/rmap.h b/include/linux/rmap.h
> index 66407434c3b5..502439f20d88 100644
> --- a/include/linux/rmap.h
> +++ b/include/linux/rmap.h
> @@ -192,7 +192,7 @@ static inline void page_dup_rmap(struct page *page, bool compound)
>  int folio_referenced(struct folio *, int is_locked,
>  			struct mem_cgroup *memcg, unsigned long *vm_flags);
>
> -void try_to_migrate(struct page *page, enum ttu_flags flags);
> +void try_to_migrate(struct folio *folio, enum ttu_flags flags);
>  void try_to_unmap(struct folio *, enum ttu_flags flags);
>
>  int make_device_exclusive_range(struct mm_struct *mm, unsigned long start,
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 4ea22b7319fd..21676a4afd07 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2294,8 +2294,8 @@ static void unmap_page(struct page *page)
>  	 * pages can simply be left unmapped, then faulted back on demand.
>  	 * If that is ever changed (perhaps for mlock), update remap_page().
>  	 */
> -	if (PageAnon(page))
> -		try_to_migrate(page, ttu_flags);
> +	if (folio_test_anon(folio))
> +		try_to_migrate(folio, ttu_flags);
>  	else
>  		try_to_unmap(folio, ttu_flags | TTU_IGNORE_MLOCK);
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 766dc67874a1..5dcdd43d983d 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -927,6 +927,7 @@ static int move_to_new_page(struct page *newpage, struct page *page,
>  static int __unmap_and_move(struct page *page, struct page *newpage,
>  				int force, enum migrate_mode mode)
>  {
> +	struct folio *folio = page_folio(page);
>  	int rc = -EAGAIN;
>  	bool page_was_mapped = false;
>  	struct anon_vma *anon_vma = NULL;
> @@ -1030,7 +1031,7 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
>  		/* Establish migration ptes */
>  		VM_BUG_ON_PAGE(PageAnon(page) && !PageKsm(page) && !anon_vma,
>  				page);
> -		try_to_migrate(page, 0);
> +		try_to_migrate(folio, 0);
>  		page_was_mapped = true;
>  	}
>
> @@ -1173,6 +1174,7 @@ static int unmap_and_move_huge_page(new_page_t get_new_page,
>  				enum migrate_mode mode, int reason,
>  				struct list_head *ret)
>  {
> +	struct folio *src = page_folio(hpage);
>  	int rc = -EAGAIN;
>  	int page_was_mapped = 0;
>  	struct page *new_hpage;
> @@ -1249,7 +1251,7 @@ static int unmap_and_move_huge_page(new_page_t get_new_page,
>  			ttu |= TTU_RMAP_LOCKED;
>  		}
>
> -		try_to_migrate(hpage, ttu);
> +		try_to_migrate(src, ttu);
>  		page_was_mapped = 1;
>
>  		if (mapping_locked)
> @@ -2449,6 +2451,7 @@ static void migrate_vma_unmap(struct migrate_vma *migrate)
>
>  	for (i = 0; i < npages; i++) {
>  		struct page *page = migrate_pfn_to_page(migrate->src[i]);
> +		struct folio *folio;
>
>  		if (!page)
>  			continue;
> @@ -2472,8 +2475,9 @@ static void migrate_vma_unmap(struct migrate_vma *migrate)
>  			put_page(page);
>  		}
>
> -		if (page_mapped(page))
> -			try_to_migrate(page, 0);
> +		folio = page_folio(page);
> +		if (folio_mapped(folio))
> +			try_to_migrate(folio, 0);
>
>  		if (page_mapped(page) || !migrate_vma_check_page(page)) {
>  			if (!is_zone_device_page(page)) {
> diff --git a/mm/rmap.c b/mm/rmap.c
> index c598fd667948..4cfac67e328c 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1767,7 +1767,7 @@ static bool try_to_migrate_one(struct page *page, struct vm_area_struct *vma,
>  	range.end = vma_address_end(&pvmw);
>  	mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma, vma->vm_mm,
>  				address, range.end);
> -	if (PageHuge(page)) {
> +	if (folio_test_hugetlb(folio)) {
>  		/*
>  		 * If sharing is possible, start and end will be adjusted
>  		 * accordingly.
> @@ -1781,21 +1781,24 @@ static bool try_to_migrate_one(struct page *page, struct vm_area_struct *vma,
>  #ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
>  		/* PMD-mapped THP migration entry */
>  		if (!pvmw.pte) {
> -			VM_BUG_ON_PAGE(PageHuge(page) ||
> -				       !PageTransCompound(page), page);
> +			subpage = folio_page(folio,
> +				pmd_pfn(*pvmw.pmd) - folio_pfn(folio));

Here you removed the assumption that folio is always <= PMD, right?

In the commit message, maybe the below is better?
In THP migration code, fixes an assumption that a folio must be <= PMD size.

> +			VM_BUG_ON_FOLIO(folio_test_hugetlb(folio) ||
> +					!folio_test_pmd_mappable(folio), folio);
>
> -			set_pmd_migration_entry(&pvmw, page);
> +			set_pmd_migration_entry(&pvmw, subpage);
>  			continue;
>  		}
>  #endif
>
>  		/* Unexpected PMD-mapped THP? */
> -		VM_BUG_ON_PAGE(!pvmw.pte, page);
> +		VM_BUG_ON_FOLIO(!pvmw.pte, folio);
>
> -		subpage = page - page_to_pfn(page) + pte_pfn(*pvmw.pte);
> +		subpage = folio_page(folio,
> +				pte_pfn(*pvmw.pte) - folio_pfn(folio));
>  		address = pvmw.address;
>
> -		if (PageHuge(page) && !PageAnon(page)) {
> +		if (folio_test_hugetlb(folio) && !folio_test_anon(folio)) {
>  			/*
>  			 * To call huge_pmd_unshare, i_mmap_rwsem must be
>  			 * held in write mode.  Caller needs to explicitly
> @@ -1833,15 +1836,15 @@ static bool try_to_migrate_one(struct page *page, struct vm_area_struct *vma,
>  		flush_cache_page(vma, address, pte_pfn(*pvmw.pte));
>  		pteval = ptep_clear_flush(vma, address, pvmw.pte);
>
> -		/* Move the dirty bit to the page. Now the pte is gone. */
> +		/* Set the dirty flag on the folio now the pte is gone. */
>  		if (pte_dirty(pteval))
> -			set_page_dirty(page);
> +			folio_mark_dirty(folio);
>
>  		/* Update high watermark before we lower rss */
>  		update_hiwater_rss(mm);
>
> -		if (is_zone_device_page(page)) {
> -			unsigned long pfn = page_to_pfn(page);
> +		if (folio_is_zone_device(folio)) {
> +			unsigned long pfn = folio_pfn(folio);
>  			swp_entry_t entry;
>  			pte_t swp_pte;
>
> @@ -1877,16 +1880,16 @@ static bool try_to_migrate_one(struct page *page, struct vm_area_struct *vma,
>  			 * changed when hugepage migrations to device private
>  			 * memory are supported.
>  			 */
> -			subpage = page;
> -		} else if (PageHWPoison(page)) {
> +			subpage = &folio->page;
> +		} else if (PageHWPoison(subpage)) {
>  			pteval = swp_entry_to_pte(make_hwpoison_entry(subpage));
> -			if (PageHuge(page)) {
> -				hugetlb_count_sub(compound_nr(page), mm);
> +			if (folio_test_hugetlb(folio)) {
> +				hugetlb_count_sub(folio_nr_pages(folio), mm);
>  				set_huge_swap_pte_at(mm, address,
>  						     pvmw.pte, pteval,
>  						     vma_mmu_pagesize(vma));
>  			} else {
> -				dec_mm_counter(mm, mm_counter(page));
> +				dec_mm_counter(mm, mm_counter(&folio->page));
>  				set_pte_at(mm, address, pvmw.pte, pteval);
>  			}
>
> @@ -1901,7 +1904,7 @@ static bool try_to_migrate_one(struct page *page, struct vm_area_struct *vma,
>  			 * migration) will not expect userfaults on already
>  			 * copied pages.
>  			 */
> -			dec_mm_counter(mm, mm_counter(page));
> +			dec_mm_counter(mm, mm_counter(&folio->page));
>  			/* We have to invalidate as we cleared the pte */
>  			mmu_notifier_invalidate_range(mm, address,
>  						      address + PAGE_SIZE);
> @@ -1947,8 +1950,8 @@ static bool try_to_migrate_one(struct page *page, struct vm_area_struct *vma,
>  		 *
>  		 * See Documentation/vm/mmu_notifier.rst
>  		 */
> -		page_remove_rmap(subpage, PageHuge(page));
> -		put_page(page);
> +		page_remove_rmap(subpage, folio_test_hugetlb(folio));
> +		folio_put(folio);
>  	}
>
>  	mmu_notifier_invalidate_range_end(&range);
> @@ -1958,13 +1961,13 @@ static bool try_to_migrate_one(struct page *page, struct vm_area_struct *vma,
>
>  /**
>   * try_to_migrate - try to replace all page table mappings with swap entries
> - * @page: the page to replace page table entries for
> + * @folio: the folio to replace page table entries for
>   * @flags: action and flags
>   *
> - * Tries to remove all the page table entries which are mapping this page and
> - * replace them with special swap entries. Caller must hold the page lock.
> + * Tries to remove all the page table entries which are mapping this folio and
> + * replace them with special swap entries. Caller must hold the folio lock.
>   */
> -void try_to_migrate(struct page *page, enum ttu_flags flags)
> +void try_to_migrate(struct folio *folio, enum ttu_flags flags)
>  {
>  	struct rmap_walk_control rwc = {
>  		.rmap_one = try_to_migrate_one,
> @@ -1981,7 +1984,7 @@ void try_to_migrate(struct page *page, enum ttu_flags flags)
>  					TTU_SYNC)))
>  		return;
>
> -	if (is_zone_device_page(page) && !is_device_private_page(page))
> +	if (folio_is_zone_device(folio) && !folio_is_device_private(folio))
>  		return;
>
>  	/*
> @@ -1992,13 +1995,13 @@ void try_to_migrate(struct page *page, enum ttu_flags flags)
>  	 * locking requirements of exec(), migration skips
>  	 * temporary VMAs until after exec() completes.
>  	 */
> -	if (!PageKsm(page) && PageAnon(page))
> +	if (!folio_test_ksm(folio) && folio_test_anon(folio))
>  		rwc.invalid_vma = invalid_migration_vma;
>
>  	if (flags & TTU_RMAP_LOCKED)
> -		rmap_walk_locked(page, &rwc);
> +		rmap_walk_locked(&folio->page, &rwc);
>  	else
> -		rmap_walk(page, &rwc);
> +		rmap_walk(&folio->page, &rwc);
>  }
>
>  /*
> -- 
> 2.34.1

Otherwise, LGTM. Thanks. Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>

--
Best Regards,
Yan, Zi

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