Re: cgroups: warning for metadata allocation with GFP_NOFAIL (was Re: folio_alloc_buffers() doing allocations > order 1 with GFP_NOFAIL)

[Matthew Wilcox <willy@infradead.org>]

On Mon, Nov 06, 2023 at 06:57:05PM -0800, Christoph Lameter wrote:
> Right.. Well lets add the cgoup folks to this.
> 
> The code that simply uses the GFP_NOFAIL to allocate cgroup metadata using
> an order > 1:
> 
> int memcg_alloc_slab_cgroups(struct slab *slab, struct kmem_cache *s,
> 				 gfp_t gfp, bool new_slab)
> {
> 	unsigned int objects = objs_per_slab(s, slab);
> 	unsigned long memcg_data;
> 	void *vec;
> 
> 	gfp &= ~OBJCGS_CLEAR_MASK;
> 	vec = kcalloc_node(objects, sizeof(struct obj_cgroup *), gfp,
> 			   slab_nid(slab));

But, but but, why does this incur an allocation larger than PAGE_SIZE?

sizeof(void *) is 8.  We have N objects allocated from the slab.  I
happen to know this is used for buffer_head, so:

buffer_head         1369   1560    104   39    1 : tunables    0    0    0 : slabdata     40     40      0

we get 39 objects per slab.  and we're only allocating one page per slab.
39 * 8 is only 312.

Maybe Christoph is playing with min_slab_order or something, so we're
getting 8 pages per slab.  That's still only 2496 bytes.  Why are we
calling into the large kmalloc path?  What's really going on here?