Re: [PATCH 4/8] migrate_pages: split unmap_and_move() to _unmap() and _move()

[Zi Yan]

On 26 Dec 2022, at 19:28, Huang Ying wrote:

> This is a preparation patch to batch the folio unmapping and moving.
>
> In this patch, unmap_and_move() is split to migrate_folio_unmap() and
> migrate_folio_move().  So, we can batch _unmap() and _move() in
> different loops later.  To pass some information between unmap and
> move, the original unused dst->mapping and dst->private are used.
>
> Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
> Cc: Zi Yan <ziy@xxxxxxxxxx>
> Cc: Yang Shi <shy828301@xxxxxxxxx>
> Cc: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> Cc: Oscar Salvador <osalvador@xxxxxxx>
> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Cc: Bharata B Rao <bharata@xxxxxxx>
> Cc: Alistair Popple <apopple@xxxxxxxxxx>
> Cc: haoxin <xhao@xxxxxxxxxxxxxxxxx>
> ---
>  include/linux/migrate.h |   1 +
>  mm/migrate.c            | 162 +++++++++++++++++++++++++++++-----------
>  2 files changed, 121 insertions(+), 42 deletions(-)
>
> diff --git a/include/linux/migrate.h b/include/linux/migrate.h
> index 3ef77f52a4f0..7376074f2e1e 100644
> --- a/include/linux/migrate.h
> +++ b/include/linux/migrate.h
> @@ -18,6 +18,7 @@ struct migration_target_control;
>   * - zero on page migration success;
>   */
>  #define MIGRATEPAGE_SUCCESS		0
> +#define MIGRATEPAGE_UNMAP		1
>
>  /**
>   * struct movable_operations - Driver page migration
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 97ea0737ab2b..e2383b430932 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1009,11 +1009,29 @@ static int move_to_new_folio(struct folio *dst, struct folio *src,
>  	return rc;
>  }
>
> -static int __unmap_and_move(struct folio *src, struct folio *dst,
> +static void __migrate_folio_record(struct folio *dst,
> +				   unsigned long page_was_mapped,
> +				   struct anon_vma *anon_vma)
> +{
> +	dst->mapping = (struct address_space *)anon_vma;
> +	dst->private = (void *)page_was_mapped;
> +}
> +
> +static void __migrate_folio_extract(struct folio *dst,
> +				   int *page_was_mappedp,
> +				   struct anon_vma **anon_vmap)
> +{
> +	*anon_vmap = (struct anon_vma *)dst->mapping;
> +	*page_was_mappedp = (unsigned long)dst->private;
> +	dst->mapping = NULL;
> +	dst->private = NULL;
> +}
> +

We probably need comments on these two functions on using dst->mapping
and dst->private. It might be hard to get the behavior later by
digging into the git log and/or looking at the code.

> +static int __migrate_folio_unmap(struct folio *src, struct folio *dst,
>  				int force, enum migrate_mode mode)
>  {
>  	int rc = -EAGAIN;
> -	bool page_was_mapped = false;
> +	int page_was_mapped = 0;
>  	struct anon_vma *anon_vma = NULL;
>  	bool is_lru = !__PageMovable(&src->page);
>
> @@ -1089,8 +1107,8 @@ static int __unmap_and_move(struct folio *src, struct folio *dst,
>  		goto out_unlock;
>
>  	if (unlikely(!is_lru)) {
> -		rc = move_to_new_folio(dst, src, mode);
> -		goto out_unlock_both;
> +		__migrate_folio_record(dst, page_was_mapped, anon_vma);
> +		return MIGRATEPAGE_UNMAP;
>  	}
>
>  	/*
> @@ -1115,11 +1133,40 @@ static int __unmap_and_move(struct folio *src, struct folio *dst,
>  		VM_BUG_ON_FOLIO(folio_test_anon(src) &&
>  			       !folio_test_ksm(src) && !anon_vma, src);
>  		try_to_migrate(src, 0);
> -		page_was_mapped = true;
> +		page_was_mapped = 1;
>  	}
>
> -	if (!folio_mapped(src))
> -		rc = move_to_new_folio(dst, src, mode);
> +	if (!folio_mapped(src)) {
> +		__migrate_folio_record(dst, page_was_mapped, anon_vma);
> +		return MIGRATEPAGE_UNMAP;
> +	}
> +
> +
> +	if (page_was_mapped)
> +		remove_migration_ptes(src, src, false);
> +
> +out_unlock_both:
> +	folio_unlock(dst);
> +out_unlock:
> +	/* Drop an anon_vma reference if we took one */
> +	if (anon_vma)
> +		put_anon_vma(anon_vma);
> +	folio_unlock(src);
> +out:
> +
> +	return rc;
> +}
> +
> +static int __migrate_folio_move(struct folio *src, struct folio *dst,
> +				enum migrate_mode mode)
> +{
> +	int rc;
> +	int page_was_mapped = 0;
> +	struct anon_vma *anon_vma = NULL;
> +
> +	__migrate_folio_extract(dst, &page_was_mapped, &anon_vma);
> +
> +	rc = move_to_new_folio(dst, src, mode);
>
>  	/*
>  	 * When successful, push dst to LRU immediately: so that if it
> @@ -1140,14 +1187,11 @@ static int __unmap_and_move(struct folio *src, struct folio *dst,
>  		remove_migration_ptes(src,
>  			rc == MIGRATEPAGE_SUCCESS ? dst : src, false);
>
> -out_unlock_both:
>  	folio_unlock(dst);
> -out_unlock:
>  	/* Drop an anon_vma reference if we took one */
>  	if (anon_vma)
>  		put_anon_vma(anon_vma);
>  	folio_unlock(src);
> -out:
>  	/*
>  	 * If migration is successful, decrease refcount of dst,
>  	 * which will not free the page because new page owner increased
> @@ -1159,19 +1203,32 @@ static int __unmap_and_move(struct folio *src, struct folio *dst,
>  	return rc;
>  }
>
> -/*
> - * Obtain the lock on folio, remove all ptes and migrate the folio
> - * to the newly allocated folio in dst.
> - */
> -static int unmap_and_move(new_page_t get_new_page,
> -				   free_page_t put_new_page,
> -				   unsigned long private, struct folio *src,
> -				   int force, enum migrate_mode mode,
> -				   enum migrate_reason reason,
> -				   struct list_head *ret)
> +static void migrate_folio_done(struct folio *src,
> +			       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(!__folio_test_movable(src)))
> +		mod_node_page_state(folio_pgdat(src), NR_ISOLATED_ANON +
> +				    folio_is_file_lru(src), -folio_nr_pages(src));
> +
> +	if (reason != MR_MEMORY_FAILURE)
> +		/* We release the page in page_handle_poison. */
> +		folio_put(src);
> +}
> +
> +/* Obtain the lock on page, remove all ptes. */
> +static int migrate_folio_unmap(new_page_t get_new_page, free_page_t put_new_page,
> +			       unsigned long private, struct folio *src,
> +			       struct folio **dstp, int force,
> +			       enum migrate_mode mode, enum migrate_reason reason,
> +			       struct list_head *ret)
>  {
>  	struct folio *dst;
> -	int rc = MIGRATEPAGE_SUCCESS;
> +	int rc = MIGRATEPAGE_UNMAP;
>  	struct page *newpage = NULL;
>
>  	if (!thp_migration_supported() && folio_test_transhuge(src))
> @@ -1182,20 +1239,50 @@ static int unmap_and_move(new_page_t get_new_page,
>  		folio_clear_active(src);
>  		folio_clear_unevictable(src);
>  		/* free_pages_prepare() will clear PG_isolated. */
> -		goto out;
> +		list_del(&src->lru);
> +		migrate_folio_done(src, reason);
> +		return MIGRATEPAGE_SUCCESS;
>  	}
>
>  	newpage = get_new_page(&src->page, private);
>  	if (!newpage)
>  		return -ENOMEM;
>  	dst = page_folio(newpage);
> +	*dstp = dst;
>
>  	dst->private = NULL;
> -	rc = __unmap_and_move(src, dst, force, mode);
> +	rc = __migrate_folio_unmap(src, dst, 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 folio to right list. */
> +	if (rc != -EAGAIN)
> +		list_move_tail(&src->lru, ret);
> +
> +	if (put_new_page)
> +		put_new_page(&dst->page, private);
> +	else
> +		folio_put(dst);
> +
> +	return rc;
> +}
> +
> +/* Migrate the folio to the newly allocated folio in dst. */
> +static int migrate_folio_move(free_page_t put_new_page, unsigned long private,
> +			      struct folio *src, struct folio *dst,
> +			      enum migrate_mode mode, enum migrate_reason reason,
> +			      struct list_head *ret)
> +{
> +	int rc;
> +
> +	rc = __migrate_folio_move(src, dst, mode);
>  	if (rc == MIGRATEPAGE_SUCCESS)
>  		set_page_owner_migrate_reason(&dst->page, reason);
>
> -out:
>  	if (rc != -EAGAIN) {
>  		/*
>  		 * A folio that has been migrated has all references
> @@ -1211,20 +1298,7 @@ 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 folios which are
> -		 * not accounted to NR_ISOLATED_*. They can be recognized
> -		 * as __folio_test_movable
> -		 */
> -		if (likely(!__folio_test_movable(src)))
> -			mod_node_page_state(folio_pgdat(src), NR_ISOLATED_ANON +
> -					folio_is_file_lru(src), -folio_nr_pages(src));
> -
> -		if (reason != MR_MEMORY_FAILURE)
> -			/*
> -			 * We release the folio in page_handle_poison.
> -			 */
> -			folio_put(src);
> +		migrate_folio_done(src, reason);
>  	} else {
>  		if (rc != -EAGAIN)
>  			list_add_tail(&src->lru, ret);
> @@ -1499,7 +1573,7 @@ static int migrate_pages_batch(struct list_head *from, new_page_t get_new_page,
>  	int pass = 0;
>  	bool is_large = false;
>  	bool is_thp = false;
> -	struct folio *folio, *folio2;
> +	struct folio *folio, *folio2, *dst = NULL;
>  	int rc, nr_pages;
>  	LIST_HEAD(split_folios);
>  	bool nosplit = (reason == MR_NUMA_MISPLACED);
> @@ -1524,9 +1598,13 @@ static int migrate_pages_batch(struct list_head *from, new_page_t get_new_page,
>
>  			cond_resched();
>
> -			rc = unmap_and_move(get_new_page, put_new_page,
> -					    private, folio, pass > 2, mode,
> -					    reason, ret_folios);
> +			rc = migrate_folio_unmap(get_new_page, put_new_page, private,
> +						 folio, &dst, pass > 2, mode,
> +						 reason, ret_folios);
> +			if (rc == MIGRATEPAGE_UNMAP)
> +				rc = migrate_folio_move(put_new_page, private,
> +							folio, dst, mode,
> +							reason, ret_folios);
>  			/*
>  			 * The rules are:
>  			 *	Success: folio will be freed
> -- 
> 2.35.1


--
Best Regards,
Yan, Zi

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