Re: [PATCH 1/6] mm: migrate: add isolate_movable_folio()

[Vishal Moola <vishal.moola@gmail.com>]

On Wed, Mar 27, 2024 at 10:10:29PM +0800, Kefeng Wang wrote:
> Like isolate_lru_page(), make isolate_movable_page() as a wrapper
> around isolate_lru_folio(), since isolate_movable_page() always
> fails on a tail page, add a warn for tail page and return immediately.
>
> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
> ---
>  include/linux/migrate.h |  3 +++
>  mm/migrate.c            | 41 +++++++++++++++++++++++------------------
>  2 files changed, 26 insertions(+), 18 deletions(-)
> 
> diff --git a/include/linux/migrate.h b/include/linux/migrate.h
> index f9d92482d117..a6c38ee7246a 100644
> --- a/include/linux/migrate.h
> +++ b/include/linux/migrate.h
> @@ -70,6 +70,7 @@ int migrate_pages(struct list_head *l, new_folio_t new, free_folio_t free,
>  		  unsigned int *ret_succeeded);
>  struct folio *alloc_migration_target(struct folio *src, unsigned long private);
>  bool isolate_movable_page(struct page *page, isolate_mode_t mode);
> +bool isolate_movable_folio(struct folio *folio, isolate_mode_t mode);
>  
>  int migrate_huge_page_move_mapping(struct address_space *mapping,
>  		struct folio *dst, struct folio *src);
> @@ -91,6 +92,8 @@ static inline struct folio *alloc_migration_target(struct folio *src,
>  	{ return NULL; }
>  static inline bool isolate_movable_page(struct page *page, isolate_mode_t mode)
>  	{ return false; }
> +static inline bool isolate_movable_folio(struct page *page, isolate_mode_t mode)
> +	{ return false; }

Wrong argument here.

>  
>  static inline int migrate_huge_page_move_mapping(struct address_space *mapping,
>  				  struct folio *dst, struct folio *src)
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 2228ca681afb..b2195b6ff32c 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -57,31 +57,29 @@
>  
>  #include "internal.h"
>  
> -bool isolate_movable_page(struct page *page, isolate_mode_t mode)
> +bool isolate_movable_folio(struct folio *folio, isolate_mode_t mode)
>  {
> -	struct folio *folio = folio_get_nontail_page(page);
>  	const struct movable_operations *mops;
>  
>  	/*
> -	 * Avoid burning cycles with pages that are yet under __free_pages(),
> +	 * Avoid burning cycles with folios that are yet under __free_pages(),
>  	 * or just got freed under us.
>  	 *
> -	 * In case we 'win' a race for a movable page being freed under us and
> +	 * In case we 'win' a race for a movable folio being freed under us and
>  	 * raise its refcount preventing __free_pages() from doing its job
> -	 * the put_page() at the end of this block will take care of
> -	 * release this page, thus avoiding a nasty leakage.
> +	 * the folio_put() at the end of this block will take care of
> +	 * release this folio, thus avoiding a nasty leakage.
>  	 */
> -	if (!folio)
> -		goto out;
> +	folio_get(folio);
>  
>  	if (unlikely(folio_test_slab(folio)))
>  		goto out_putfolio;
>  	/* Pairs with smp_wmb() in slab freeing, e.g. SLUB's __free_slab() */
>  	smp_rmb();
>  	/*
> -	 * Check movable flag before taking the page lock because
> -	 * we use non-atomic bitops on newly allocated page flags so
> -	 * unconditionally grabbing the lock ruins page's owner side.
> +	 * Check movable flag before taking the folio lock because
> +	 * we use non-atomic bitops on newly allocated folio flags so
> +	 * unconditionally grabbing the lock ruins folio's owner side.
>  	 */
>  	if (unlikely(!__folio_test_movable(folio)))
>  		goto out_putfolio;
> @@ -91,13 +89,13 @@ bool isolate_movable_page(struct page *page, isolate_mode_t mode)
>  		goto out_putfolio;
>  
>  	/*
> -	 * As movable pages are not isolated from LRU lists, concurrent
> -	 * compaction threads can race against page migration functions
> -	 * as well as race against the releasing a page.
> +	 * As movable folios are not isolated from LRU lists, concurrent
> +	 * compaction threads can race against folio migration functions
> +	 * as well as race against the releasing a folio.
>  	 *
> -	 * In order to avoid having an already isolated movable page
> +	 * In order to avoid having an already isolated movable folio
>  	 * being (wrongly) re-isolated while it is under migration,
> -	 * or to avoid attempting to isolate pages being released,
> +	 * or to avoid attempting to isolate folios being released,
>  	 * lets be sure we have the page lock
>  	 * before proceeding with the movable page isolation steps.
>  	 */
> @@ -113,7 +111,7 @@ bool isolate_movable_page(struct page *page, isolate_mode_t mode)
>  	if (!mops->isolate_page(&folio->page, mode))
>  		goto out_no_isolated;
>  
> -	/* Driver shouldn't use PG_isolated bit of page->flags */
> +	/* Driver shouldn't use PG_isolated bit of folio->flags */
>  	WARN_ON_ONCE(folio_test_isolated(folio));
>  	folio_set_isolated(folio);
>  	folio_unlock(folio);
> @@ -124,10 +122,17 @@ bool isolate_movable_page(struct page *page, isolate_mode_t mode)
>  	folio_unlock(folio);
>  out_putfolio:
>  	folio_put(folio);
> -out:
>  	return false;
>  }
>  
> +bool isolate_movable_page(struct page *page, isolate_mode_t mode)
> +{
> +	if (WARN_RATELIMIT(PageTail(page), "trying to isolate tail page"))
> +		return false;

This warning doesn't make sense. As of now, we still isolate_movable_page()
to be able to take in a tail page, we just don't want to operate on it.

> +	return isolate_movable_folio((struct folio *)page, mode);
> +}
> +
>  static void putback_movable_folio(struct folio *folio)
>  {
>  	const struct movable_operations *mops = folio_movable_ops(folio);
> -- 
> 2.27.0
>