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

[Matthew Wilcox <willy@infradead.org>]

On Mon, Nov 24, 2025 at 11:31:06AM +0900, Harry Yoo wrote:
> >  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;

We could!  However, I decided that we had these extra warnings to catch
some uses of ksize() on non-kmalloc-large objects, and so they were
really caught by the new:

+       if (WARN_ON(!slab))
+               return page_size(page);

Now, you could argue that we're now not warning for ksize(kmalloc() + N)
for N in the range 1..4095.  But I'm not sure it's particularly useful
to be warning for those cases.

I suppose size <= KMALLOC_MAX_CACHE_SIZE would also catch an internal
bug in kmalloc where we set PageLargeKmalloc on a small allocation,
but given how few places actually call ksize(), I doubt this would be
a useful self-check either.