Re: [PATCH v4 14/16] memcg: Convert mem_cgroup_from_obj_folio() to mem_cgroup_from_obj_slab()

[Matthew Wilcox <willy@infradead.org>]

On Thu, Nov 13, 2025 at 11:42:01AM -0800, Shakeel Butt wrote:
> On Thu, Nov 13, 2025 at 05:28:59PM +0100, Vlastimil Babka wrote:
> > > E.g. !vmap kernel stack pages -> mod_lruvec_kmem_state -> mem_cgroup_from_obj_slab
> > > 
> > > How about:
> > > 
> > > 	if ((slab = virt_to_slap(p)))
> > > 		return mem_cgroup_from_obj_slab(slab, p);
> > > 	return folio_memcg_check(virt_to_folio(p), p);
> > 
> > page_memcg_check() maybe instead? we shouldn't get a tail page here, no?
> 
> Do you mean page_memcg_check(virt_to_page(p), p)? But virt_to_page(p)
> can return tail page, right?

Only if it's legitimate to call mod_lruvec_kmem_state() with "a pointer
to somewhere inside the object" rather than "a pointer to the object".

For example, it's legitimate to call copy_to_user() with a pointer
somewhere inside the object, so the usercopy code has to handle that
case.  But it's only legitimate to call kfree() with a pointer that's
at the start of an object, so kfree() can cheerfully BUG_ON()
PageTail(virt_to_page(p)).