Re: [PATCH 2/3] mm: use folio_headpage() instead of folio_page()

[Matthew Wilcox]

On Fri, Jan 06, 2023 at 05:40:27PM +0000, SeongJae Park wrote:
> diff --git a/mm/shmem.c b/mm/shmem.c
> index bc5c156ef470..8ae73973a7fc 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -3211,7 +3211,7 @@ static const char *shmem_get_link(struct dentry *dentry,
>  		folio = filemap_get_folio(inode->i_mapping, 0);
>  		if (!folio)
>  			return ERR_PTR(-ECHILD);
> -		if (PageHWPoison(folio_page(folio, 0)) ||
> +		if (PageHWPoison(folio_headpage(folio)) ||

This is actually incorrect.  We don't want the head page, we want the
page at index 0.  It's a subtle but important difference later on.

> @@ -3222,7 +3222,7 @@ static const char *shmem_get_link(struct dentry *dentry,
>  			return ERR_PTR(error);
>  		if (!folio)
>  			return ERR_PTR(-ECHILD);
> -		if (PageHWPoison(folio_page(folio, 0))) {
> +		if (PageHWPoison(folio_headpage(folio))) {

Same here.

> +++ b/mm/slab.c
> @@ -1373,7 +1373,7 @@ static struct slab *kmem_getpages(struct kmem_cache *cachep, gfp_t flags,
>  	/* Make the flag visible before any changes to folio->mapping */
>  	smp_wmb();
>  	/* Record if ALLOC_NO_WATERMARKS was set when allocating the slab */
> -	if (sk_memalloc_socks() && page_is_pfmemalloc(folio_page(folio, 0)))
> +	if (sk_memalloc_socks() && page_is_pfmemalloc(folio_headpage(folio)))

We should have a folio_is_pfmemalloc().

> @@ -1389,7 +1389,7 @@ static void kmem_freepages(struct kmem_cache *cachep, struct slab *slab)
>  
>  	BUG_ON(!folio_test_slab(folio));
>  	__slab_clear_pfmemalloc(slab);
> -	page_mapcount_reset(folio_page(folio, 0));
> +	page_mapcount_reset(folio_headpage(folio));

This one should be &folio->page.

> @@ -1398,7 +1398,7 @@ static void kmem_freepages(struct kmem_cache *cachep, struct slab *slab)
>  	if (current->reclaim_state)
>  		current->reclaim_state->reclaimed_slab += 1 << order;
>  	unaccount_slab(slab, order, cachep);
> -	__free_pages(folio_page(folio, 0), order);
> +	__free_pages(folio_headpage(folio), order);

&folio->page.

> @@ -939,9 +939,9 @@ void free_large_kmalloc(struct folio *folio, void *object)
>  	kasan_kfree_large(object);
>  	kmsan_kfree_large(object);
>  
> -	mod_lruvec_page_state(folio_page(folio, 0), NR_SLAB_UNRECLAIMABLE_B,
> +	mod_lruvec_page_state(folio_headpage(folio), NR_SLAB_UNRECLAIMABLE_B,
>  			      -(PAGE_SIZE << order));

	lruvec_stat_mod_folio(folio, NR_SLAB_UNRECLAIMABLE_B, ...

> -	__free_pages(folio_page(folio, 0), order);
> +	__free_pages(folio_headpage(folio), order);

&folio->page.

> +++ b/mm/slub.c
> @@ -1859,7 +1859,7 @@ static inline struct slab *alloc_slab_page(gfp_t flags, int node,
>  	__folio_set_slab(folio);
>  	/* Make the flag visible before any changes to folio->mapping */
>  	smp_wmb();
> -	if (page_is_pfmemalloc(folio_page(folio, 0)))
> +	if (page_is_pfmemalloc(folio_headpage(folio)))

folio_is_pfmemalloc()

> @@ -2066,7 +2066,7 @@ static void __free_slab(struct kmem_cache *s, struct slab *slab)
>  	if (current->reclaim_state)
>  		current->reclaim_state->reclaimed_slab += pages;
>  	unaccount_slab(slab, order, s);
> -	__free_pages(folio_page(folio, 0), order);
> +	__free_pages(folio_headpage(folio), order);

&folio->page.