Re: [RFC 2/6] mm/migrate_pages: split unmap_and_move() to _unmap() and _move()

Alistair Popple
2022-09-26T09:40:30Z

Huang Ying <ying.huang@intel.com> writes:

> This is a preparation patch to batch the page unmapping and moving
> for the normal pages and THP.
>
> In this patch, unmap_and_move() is split to migrate_page_unmap() and
> migrate_page_move(). So, we can batch _unmap() and _move() in
> different loops later. To pass some information between unmap and
> move, the original unused newpage->mapping and newpage->private are
> used.

This looks like it could cause a deadlock between two threads migrating
the same pages if force == true && mode != MIGRATE_ASYNC as
migrate_page_unmap() will call lock_page() while holding the lock on
other pages in the list. Therefore the two threads could deadlock if the
pages are in a different order.

> Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
> Cc: Zi Yan <ziy@nvidia.com>
> Cc: Yang Shi <shy828301@gmail.com>
> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
> Cc: Oscar Salvador <osalvador@suse.de>
> Cc: Matthew Wilcox <willy@infradead.org>
> ---
> mm/migrate.c | 164 ++++++++++++++++++++++++++++++++++++++-------------
> 1 file changed, 122 insertions(+), 42 deletions(-)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 117134f1c6dc..4a81e0bfdbcd 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -976,13 +976,32 @@ static int move_to_new_folio(struct folio *dst, struct folio *src,
> return rc;
> }
>
> -static int __unmap_and_move(struct page *page, struct page *newpage,
> +static void __migrate_page_record(struct page *newpage,
> + int page_was_mapped,
> + struct anon_vma *anon_vma)
> +{
> + newpage->mapping = (struct address_space *)anon_vma;
> + newpage->private = page_was_mapped;
> +}
> +
> +static void __migrate_page_extract(struct page *newpage,
> + int *page_was_mappedp,
> + struct anon_vma **anon_vmap)
> +{
> + *anon_vmap = (struct anon_vma *)newpage->mapping;
> + *page_was_mappedp = newpage->private;
> + newpage->mapping = NULL;
> + newpage->private = 0;
> +}
> +
> +#define MIGRATEPAGE_UNMAP 1
> +
> +static int __migrate_page_unmap(struct page *page, struct page *newpage,
> int force, enum migrate_mode mode)
> {
> struct folio *folio = page_folio(page);
> - struct folio *dst = page_folio(newpage);
> int rc = -EAGAIN;
> - bool page_was_mapped = false;
> + int page_was_mapped = 0;
> struct anon_vma *anon_vma = NULL;
> bool is_lru = !__PageMovable(page);
>
> @@ -1058,8 +1077,8 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
> goto out_unlock;
>
> if (unlikely(!is_lru)) {
> - rc = move_to_new_folio(dst, folio, mode);
> - goto out_unlock_both;
> + __migrate_page_record(newpage, page_was_mapped, anon_vma);
> + return MIGRATEPAGE_UNMAP;
> }
>
> /*
> @@ -1085,11 +1104,41 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
> VM_BUG_ON_PAGE(PageAnon(page) && !PageKsm(page) && !anon_vma,
> page);
> try_to_migrate(folio, 0);
> - page_was_mapped = true;
> + page_was_mapped = 1;
> + }
> +
> + if (!page_mapped(page)) {
> + __migrate_page_record(newpage, page_was_mapped, anon_vma);
> + return MIGRATEPAGE_UNMAP;
> }
>
> - if (!page_mapped(page))
> - rc = move_to_new_folio(dst, folio, mode);
> + if (page_was_mapped)
> + remove_migration_ptes(folio, folio, false);
> +
> +out_unlock_both:
> + unlock_page(newpage);
> +out_unlock:
> + /* Drop an anon_vma reference if we took one */
> + if (anon_vma)
> + put_anon_vma(anon_vma);
> + unlock_page(page);
> +out:
> +
> + return rc;
> +}
> +
> +static int __migrate_page_move(struct page *page, struct page *newpage,
> + enum migrate_mode mode)
> +{
> + struct folio *folio = page_folio(page);
> + struct folio *dst = page_folio(newpage);
> + int rc;
> + int page_was_mapped = 0;
> + struct anon_vma *anon_vma = NULL;
> +
> + __migrate_page_extract(newpage, &page_was_mapped, &anon_vma);
> +
> + rc = move_to_new_folio(dst, folio, mode);
>
> /*
> * When successful, push newpage to LRU immediately: so that if it
> @@ -1110,14 +1159,11 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
> remove_migration_ptes(folio,
> rc == MIGRATEPAGE_SUCCESS ? dst : folio, false);
>
> -out_unlock_both:
> unlock_page(newpage);
> -out_unlock:
> /* Drop an anon_vma reference if we took one */
> if (anon_vma)
> put_anon_vma(anon_vma);
> unlock_page(page);
> -out:
> /*
> * If migration is successful, decrease refcount of the newpage,
> * which will not free the page because new page owner increased
> @@ -1129,18 +1175,31 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
> return rc;
> }
>
> -/*
> - * Obtain the lock on page, remove all ptes and migrate the page
> - * to the newly allocated page in newpage.
> - */
> -static int unmap_and_move(new_page_t get_new_page,
> - free_page_t put_new_page,
> - unsigned long private, struct page *page,
> - int force, enum migrate_mode mode,
> - enum migrate_reason reason,
> - struct list_head *ret)
> +static void migrate_page_done(struct page *page,
> + enum migrate_reason reason)
> +{
> + /*
> + * Compaction can migrate also non-LRU pages which are
> + * not accounted to NR_ISOLATED_*. They can be recognized
> + * as __PageMovable
> + */
> + if (likely(!__PageMovable(page)))
> + mod_node_page_state(page_pgdat(page), NR_ISOLATED_ANON +
> + page_is_file_lru(page), -thp_nr_pages(page));
> +
> + if (reason != MR_MEMORY_FAILURE)
> + /* We release the page in page_handle_poison. */
> + put_page(page);
> +}
> +
> +/* Obtain the lock on page, remove all ptes. */
> +static int migrate_page_unmap(new_page_t get_new_page, free_page_t put_new_page,
> + unsigned long private, struct page *page,
> + struct page **newpagep, int force,
> + enum migrate_mode mode, enum migrate_reason reason,
> + struct list_head *ret)
> {
> - int rc = MIGRATEPAGE_SUCCESS;
> + int rc = MIGRATEPAGE_UNMAP;
> struct page *newpage = NULL;
>
> if (!thp_migration_supported() && PageTransHuge(page))
> @@ -1151,19 +1210,48 @@ static int unmap_and_move(new_page_t get_new_page,
> ClearPageActive(page);
> ClearPageUnevictable(page);
> /* free_pages_prepare() will clear PG_isolated. */
> - goto out;
> + list_del(&page->lru);
> + migrate_page_done(page, reason);
> + return MIGRATEPAGE_SUCCESS;
> }
>
> newpage = get_new_page(page, private);
> if (!newpage)
> return -ENOMEM;
> + *newpagep = newpage;
>
> - newpage->private = 0;
> - rc = __unmap_and_move(page, newpage, force, mode);
> + rc = __migrate_page_unmap(page, newpage, force, mode);
> + if (rc == MIGRATEPAGE_UNMAP)
> + return rc;
> +
> + /*
> + * A page that has not been migrated will have kept its
> + * references and be restored.
> + */
> + /* restore the page to right list. */
> + if (rc != -EAGAIN)
> + list_move_tail(&page->lru, ret);
> +
> + if (put_new_page)
> + put_new_page(newpage, private);
> + else
> + put_page(newpage);
> +
> + return rc;
> +}
> +
> +/* Migrate the page to the newly allocated page in newpage. */
> +static int migrate_page_move(free_page_t put_new_page, unsigned long private,
> + struct page *page, struct page *newpage,
> + enum migrate_mode mode, enum migrate_reason reason,
> + struct list_head *ret)
> +{
> + int rc;
> +
> + rc = __migrate_page_move(page, newpage, mode);
> if (rc == MIGRATEPAGE_SUCCESS)
> set_page_owner_migrate_reason(newpage, reason);
>
> -out:
> if (rc != -EAGAIN) {
> /*
> * A page that has been migrated has all references
> @@ -1179,20 +1267,7 @@ static int unmap_and_move(new_page_t get_new_page,
> * we want to retry.
> */
> if (rc == MIGRATEPAGE_SUCCESS) {
> - /*
> - * Compaction can migrate also non-LRU pages which are
> - * not accounted to NR_ISOLATED_*. They can be recognized
> - * as __PageMovable
> - */
> - if (likely(!__PageMovable(page)))
> - mod_node_page_state(page_pgdat(page), NR_ISOLATED_ANON +
> - page_is_file_lru(page), -thp_nr_pages(page));
> -
> - if (reason != MR_MEMORY_FAILURE)
> - /*
> - * We release the page in page_handle_poison.
> - */
> - put_page(page);
> + migrate_page_done(page, reason);
> } else {
> if (rc != -EAGAIN)
> list_add_tail(&page->lru, ret);
> @@ -1405,6 +1480,7 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
> int pass = 0;
> bool is_thp = false;
> struct page *page;
> + struct page *newpage = NULL;
> struct page *page2;
> int rc, nr_subpages;
> LIST_HEAD(ret_pages);
> @@ -1493,9 +1569,13 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
> if (PageHuge(page))
> continue;
>
> - rc = unmap_and_move(get_new_page, put_new_page,
> - private, page, pass > 2, mode,
> + rc = migrate_page_unmap(get_new_page, put_new_page, private,
> + page, &newpage, pass > 2, mode,
> reason, &ret_pages);
> + if (rc == MIGRATEPAGE_UNMAP)
> + rc = migrate_page_move(put_new_page, private,
> + page, newpage, mode,
> + reason, &ret_pages);
> /*
> * The rules are:
> * Success: page will be freed