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?
