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From:   Alistair Popple <apopple@nvidia.com>
To:     Huang Ying <ying.huang@intel.com>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Zi Yan <ziy@nvidia.com>, Yang Shi <shy828301@gmail.com>,
        Baolin Wang <baolin.wang@linux.alibaba.com>,
        Oscar Salvador <osalvador@suse.de>,
        Matthew Wilcox <willy@infradead.org>
Subject: Re: [RFC 2/6] mm/migrate_pages: split unmap_and_move() to _unmap()
 and _move()
Date:   Mon, 26 Sep 2022 19:28:42 +1000
In-reply-to: <20220921060616.73086-3-ying.huang@intel.com>
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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