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

[Kent Overstreet <kent.overstreet@linux.dev>]

On Fri, Jul 11, 2025 at 05:58:23PM +0900, Harry Yoo wrote:
> 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 <vitaly.wool@konsulko.se>
> > ---
> >  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?

You generally need size to be a multiple of align...