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

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

On Mon, Nov 24, 2025 at 04:28:20AM +0000, Matthew Wilcox wrote:
> 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);

That makes sense!

> 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.

Yeah they are a bit paranoid and I don't have enough evidence to argue
that they'll be particularly useful..

Looks like my nitpicking failed ;)
Thanks!

-- 
Cheers,
Harry / Hyeonggon