Re: [PATCH 1/2] mm, slab: use frozen pages for large kmalloc

[Harry Yoo <harry.yoo@oracle.com>]

On Thu, May 29, 2025 at 10:56:26AM +0200, Vlastimil Babka wrote:
> Since slab pages are now frozen, it makes sense to have large kmalloc()
> objects behave same as small kmalloc(), as the choice between the two is
> an implementation detail depending on allocation size.
> 
> Notably, increasing refcount on a slab page containing kmalloc() object
> is not possible anymore, so it should be consistent for large kmalloc
> pages.
> 
> Therefore, change large kmalloc to use the frozen pages API.
> 
> Because of some unexpected fallout in the slab pages case (see commit
> b9c0e49abfca ("mm: decline to manipulate the refcount on a slab page"),
> implement the same kind of checks and warnings as part of this change.
> 
> Notably, networking code using sendpage_ok() to determine whether the
> page refcount can be manipulated in the network stack should continue
> behaving correctly. Before this change, the function returns true for
> large kmalloc pages and page refcount can be manipulated. After this
> change, the function will return false.
> 
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> ---

Acked-by: Harry Yoo <harry.yoo@oracle.com>

-- 
Cheers,
Harry / Hyeonggon

>  include/linux/mm.h | 4 +++-
>  mm/slub.c          | 7 +++++--
>  2 files changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index bf55206935c467f7508e863332063bb15f904a24..d3eb6adf9fa949fbd611470182a03c743b16aac7 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1549,6 +1549,8 @@ static inline void get_page(struct page *page)
>  	struct folio *folio = page_folio(page);
>  	if (WARN_ON_ONCE(folio_test_slab(folio)))
>  		return;
> +	if (WARN_ON_ONCE(folio_test_large_kmalloc(folio)))
> +		return;
>  	folio_get(folio);
>  }
>  
> @@ -1643,7 +1645,7 @@ static inline void put_page(struct page *page)
>  {
>  	struct folio *folio = page_folio(page);
>  
> -	if (folio_test_slab(folio))
> +	if (folio_test_slab(folio) || folio_test_large_kmalloc(folio))
>  		return;
>  
>  	folio_put(folio);
> diff --git a/mm/slub.c b/mm/slub.c
> index dc9e729e1d269b5d362cb5bc44f824640ffd00f3..d7a62063a1676a327e13536bf724f0160f1fc8dc 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -4281,8 +4281,11 @@ static void *___kmalloc_large_node(size_t size, gfp_t flags, int node)
>  	if (unlikely(flags & GFP_SLAB_BUG_MASK))
>  		flags = kmalloc_fix_flags(flags);
>  
> +	if (node == NUMA_NO_NODE)
> +		node = numa_mem_id();
> +
>  	flags |= __GFP_COMP;
> -	folio = (struct folio *)alloc_pages_node_noprof(node, flags, order);
> +	folio = (struct folio *)__alloc_frozen_pages_noprof(flags, order, node, NULL);
>  	if (folio) {
>  		ptr = folio_address(folio);
>  		lruvec_stat_mod_folio(folio, NR_SLAB_UNRECLAIMABLE_B,
> @@ -4778,7 +4781,7 @@ static void free_large_kmalloc(struct folio *folio, void *object)
>  	lruvec_stat_mod_folio(folio, NR_SLAB_UNRECLAIMABLE_B,
>  			      -(PAGE_SIZE << order));
>  	__folio_clear_large_kmalloc(folio);
> -	folio_put(folio);
> +	free_frozen_pages(&folio->page, order);
>  }
>  
>  /*
> 
> -- 
> 2.49.0
>