Re: [PATCH v4 02/16] slab: Remove folio references from __ksize()

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

On Thu, Nov 13, 2025 at 12:09:16AM +0000, Matthew Wilcox (Oracle) wrote:
> In the future, we will separate slab, folio and page from each other
> and calling virt_to_folio() on an address allocated from slab will
> return NULL.  Delay the conversion from struct page to struct slab
> until we know we're not dealing with a large kmalloc allocation.
> There's a minor win for large kmalloc allocations as we avoid the
> compound_head() hidden in virt_to_folio().
> 
> This deprecates calling ksize() on memory allocated by alloc_pages().
> Today it becomes a warning and support will be removed entirely in
> the future.
> 
> Introduce large_kmalloc_size() to abstract how we represent the size
> of a large kmalloc allocation.  For now, this is the same as
> page_size(), but it will change with separately allocated memdescs.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---

Looks good to me,
Reviewed-by: Harry Yoo <harry.yoo@oracle.com>

With a minor comment below.

>  include/linux/page-flags.h |  2 +-
>  mm/slab.h                  | 10 ++++++++++
>  mm/slab_common.c           | 23 ++++++++++++-----------
>  3 files changed, 23 insertions(+), 12 deletions(-)
> 
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index d2824daa98cf..236b4e25fce0 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -997,26 +997,27 @@ void __init create_kmalloc_caches(void)
>   */
>  size_t __ksize(const void *object)
>  {
> -	struct folio *folio;
> +	const struct page *page;
> +	const struct slab *slab;
>  
>  	if (unlikely(object == ZERO_SIZE_PTR))
>  		return 0;
>  
> -	folio = virt_to_folio(object);
> +	page = virt_to_page(object);
>  
> -	if (unlikely(!folio_test_slab(folio))) {
> -		if (WARN_ON(folio_size(folio) <= KMALLOC_MAX_CACHE_SIZE))
> -			return 0;
> -		if (WARN_ON(object != folio_address(folio)))
> -			return 0;

nit: we can still have these sanity checks even when we're not converting it
to folio?

size_t size = large_kmalloc_size(page);
if (WARN_ON(size <= KMALLOC_MAX_CACHE_SIZE))
    return 0;
if (WARN_ON(object & (size - 1))
    return 0;
return size;

-- 
Cheers,
Harry / Hyeonggon