Re: [PATCH RFC 01/14] mm: memcg: subpage charging API

[Johannes Weiner]

On Thu, Sep 05, 2019 at 02:45:45PM -0700, Roman Gushchin wrote:
> Introduce an API to charge subpage objects to the memory cgroup.
> The API will be used by the new slab memory controller. Later it
> can also be used to implement percpu memory accounting.
> In both cases, a single page can be shared between multiple cgroups
> (and in percpu case a single allocation is split over multiple pages),
> so it's not possible to use page-based accounting.
> 
> The implementation is based on percpu stocks. Memory cgroups are still
> charged in pages, and the residue is stored in perpcu stock, or on the
> memcg itself, when it's necessary to flush the stock.

Did you just implement a slab allocator for page_counter to track
memory consumed by the slab allocator?

> @@ -2500,8 +2577,9 @@ void mem_cgroup_handle_over_high(void)
>  }
>  
>  static int try_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
> -		      unsigned int nr_pages)
> +		      unsigned int amount, bool subpage)
>  {
> +	unsigned int nr_pages = subpage ? ((amount >> PAGE_SHIFT) + 1) : amount;
>  	unsigned int batch = max(MEMCG_CHARGE_BATCH, nr_pages);
>  	int nr_retries = MEM_CGROUP_RECLAIM_RETRIES;
>  	struct mem_cgroup *mem_over_limit;
> @@ -2514,7 +2592,9 @@ static int try_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
>  	if (mem_cgroup_is_root(memcg))
>  		return 0;
>  retry:
> -	if (consume_stock(memcg, nr_pages))
> +	if (subpage && consume_subpage_stock(memcg, amount))
> +		return 0;
> +	else if (!subpage && consume_stock(memcg, nr_pages))
>  		return 0;

The layering here isn't clean. We have an existing per-cpu cache to
batch-charge the page counter. Why does the new subpage allocator not
sit on *top* of this, instead of wedged in between?

I think what it should be is a try_charge_bytes() that simply gets one
page from try_charge() and then does its byte tracking, regardless of
how try_charge() chooses to implement its own page tracking.

That would avoid the awkward @amount + @subpage multiplexing, as well
as annotating all existing callsites of try_charge() with a
non-descript "false" parameter.

You can still reuse the stock data structures, use the lower bits of
stock->nr_bytes for a different cgroup etc., but the charge API should
really be separate.