On Tue, Nov 07, 2023 at 07:24:08PM +0000, Matthew Wilcox wrote:
> 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?
Good question and I *guess* it's something related to Christoph's hardware
(64k pages or something like this) - otherwise we would see it sooner.
I'd like to have the answer too.
Thanks!
