Re: [PATCH v12 2/4] mm/slub: allow to set node and align in k[v]realloc

On Wed, Jul 09, 2025 at 07:24:41PM +0200, Vitaly Wool wrote:
> Reimplement k[v]realloc_node() to be able to set node and
> alignment should a user need to do so. In order to do that while
> retaining the maximal backward compatibility, add
> k[v]realloc_node_align() functions and redefine the rest of API
> using these new ones.
> 
> While doing that, we also keep the number of  _noprof variants to a
> minimum, which implies some changes to the existing users of older
> _noprof functions, that basically being bcachefs.
> 
> With that change we also provide the ability for the Rust part of
> the kernel to set node and alignment in its K[v]xxx
> [re]allocations.
> 
> Signed-off-by: Vitaly Wool <[email protected]>
> ---
>  fs/bcachefs/darray.c   |  2 +-
>  fs/bcachefs/util.h     |  2 +-
>  include/linux/bpfptr.h |  2 +-
>  include/linux/slab.h   | 38 +++++++++++++++----------
>  lib/rhashtable.c       |  4 +--
>  mm/slub.c              | 64 +++++++++++++++++++++++++++++-------------
>  6 files changed, 72 insertions(+), 40 deletions(-)
 
> diff --git a/mm/slub.c b/mm/slub.c
> index c4b64821e680..6fad4cdea6c4 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -4845,7 +4845,7 @@ void kfree(const void *object)
>  EXPORT_SYMBOL(kfree);
>  
>  static __always_inline __realloc_size(2) void *
> -__do_krealloc(const void *p, size_t new_size, gfp_t flags)
> +__do_krealloc(const void *p, size_t new_size, unsigned long align, gfp_t flags, int nid)
>  {
>  	void *ret;
>  	size_t ks = 0;
> @@ -4859,6 +4859,20 @@ __do_krealloc(const void *p, size_t new_size, gfp_t flags)
>  	if (!kasan_check_byte(p))
>  		return NULL;
>  
> +	/* refuse to proceed if alignment is bigger than what kmalloc() provides */
> +	if (!IS_ALIGNED((unsigned long)p, align) || new_size < align)
> +		return NULL;

Hmm but what happens if `p` is aligned to `align`, but the new object is not?

For example, what will happen if we  allocate object with size=64, align=64
and then do krealloc with size=96, align=64...

Or am I missing something?

> +	/*
> +	 * If reallocation is not necessary (e. g. the new size is less
> +	 * than the current allocated size), the current allocation will be
> +	 * preserved unless __GFP_THISNODE is set. In the latter case a new
> +	 * allocation on the requested node will be attempted.
> +	 */
> +	if (unlikely(flags & __GFP_THISNODE) && nid != NUMA_NO_NODE &&
> +		     nid != page_to_nid(virt_to_page(p)))
> +		goto alloc_new;
> +
>  	if (is_kfence_address(p)) {
>  		ks = orig_size = kfence_ksize(p);
>  	} else {

-- 
Cheers,
Harry / Hyeonggon