Re: [RFC PATCH 09/14] mm: Handle large folios in free_unref_folios()

[Ryan Roberts <ryan.roberts@arm.com>]

On 25/08/2023 14:59, Matthew Wilcox (Oracle) wrote:
> Call folio_undo_large_rmappable() if needed.  free_unref_page_prepare()
> destroys the ability to call folio_order(), so stash the order in
> folio->private for the benefit of the second loop.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---
>  mm/page_alloc.c | 21 +++++++++++++++------
>  1 file changed, 15 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index bca5c70b5576..e586d17fb7f2 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2465,7 +2465,7 @@ void free_unref_page(struct page *page, unsigned int order)
>  }
>  
>  /*
> - * Free a batch of 0-order pages
> + * Free a batch of folios
>   */
>  void free_unref_folios(struct folio_batch *folios)
>  {
> @@ -2478,7 +2478,11 @@ void free_unref_folios(struct folio_batch *folios)
>  	for (i = 0, j = 0; i < folios->nr; i++) {
>  		struct folio *folio = folios->folios[i];
>  		unsigned long pfn = folio_pfn(folio);
> -		if (!free_unref_page_prepare(&folio->page, pfn, 0))
> +		unsigned int order = folio_order(folio);

Do you need to do anything special for hugetlb folios? I see that
destroy_large_folio() has:

	if (folio_test_hugetlb(folio)) {
		free_huge_folio(folio);
		return;
	}

> +
> +		if (order > 0 && folio_test_large_rmappable(folio))
> +			folio_undo_large_rmappable(folio);
> +		if (!free_unref_page_prepare(&folio->page, pfn, order))
>  			continue;
>  
>  		/*
> @@ -2486,11 +2490,13 @@ void free_unref_folios(struct folio_batch *folios)
>  		 * comment in free_unref_page.
>  		 */
>  		migratetype = get_pcppage_migratetype(&folio->page);
> -		if (unlikely(is_migrate_isolate(migratetype))) {
> +		if (order > PAGE_ALLOC_COSTLY_ORDER ||

Should this be `if (!pcp_allowed_order(order) ||` ? That helper includes the THP
pageblock_order too.

> +		    is_migrate_isolate(migratetype)) {
>  			free_one_page(folio_zone(folio), &folio->page, pfn,
> -					0, migratetype, FPI_NONE);
> +					order, migratetype, FPI_NONE);
>  			continue;
>  		}
> +		folio->private = (void *)(unsigned long)order;
>  		if (j != i)
>  			folios->folios[j] = folio;
>  		j++;
> @@ -2500,7 +2506,9 @@ void free_unref_folios(struct folio_batch *folios)
>  	for (i = 0; i < folios->nr; i++) {
>  		struct folio *folio = folios->folios[i];
>  		struct zone *zone = folio_zone(folio);
> +		unsigned int order = (unsigned long)folio->private;
>  
> +		folio->private = NULL;
>  		migratetype = get_pcppage_migratetype(&folio->page);
>  
>  		/* Different zone requires a different pcp lock */
> @@ -2519,7 +2527,7 @@ void free_unref_folios(struct folio_batch *folios)
>  			if (unlikely(!pcp)) {
>  				pcp_trylock_finish(UP_flags);
>  				free_one_page(zone, &folio->page,
> -						folio_pfn(folio), 0,
> +						folio_pfn(folio), order,
>  						migratetype, FPI_NONE);
>  				locked_zone = NULL;
>  				continue;
> @@ -2535,7 +2543,8 @@ void free_unref_folios(struct folio_batch *folios)
>  			migratetype = MIGRATE_MOVABLE;
>  
>  		trace_mm_page_free_batched(&folio->page);
> -		free_unref_page_commit(zone, pcp, &folio->page, migratetype, 0);
> +		free_unref_page_commit(zone, pcp, &folio->page, migratetype,
> +				order);
>  	}
>  
>  	if (pcp) {