Re: [PATCH v5 3/7] mm/page_counter: introduce per-page_counter stock

[Shakeel Butt]

On Mon, Aug 31, 2026 at 09:37:47AM -0700, Joshua Hahn wrote:
> In order to avoid expensive hierarchy walks on every memcg charge and
> limit check, memcontrol uses per-cpu stocks (memcg_stock_pcp) to cache
> pre-charged pages and introduce a fast path to try_charge_memcg.
> 
> However, there are a few quirks with the current implementation that
> can be improved upon.
> 
> First, each memcg_stock_pcp can only cache the charges of 7 memcgs
> (NR_MEMCG_STOCK). When an 8th memcg wants to cache its charge on a CPU,
> a victim memcg is chosen among the 7 cached memcgs and is evicted,
> losing all cached charges.
> 
> Second, stock draining is per-CPU rather than per-memcg. That is,
> when a memcg is under pressure and must retrieve all cached charges,
> it iterates through every CPU and drains the stock charges of all
> present memcgs. This means that one under-pressure memcg evicts the
> caches of all co-cpu-resident memcg stock caches.
> 
> Finally, stock is tightly coupled with memcg, so adding new
> page_counters to memcg is an unscalable operation where only one counter
> gets to use the fastpath.
> 
> We can address all of these concerns by pushing stock caches down to the
> page_counter level, and making each counter responsible for its own
> charge.
> 
> Introduce struct page_counter_stock along with its allocation, free, and
> per-CPU drain helpers.
> 
> No functional change intended.
> 
> Suggested-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> Signed-off-by: Joshua Hahn <joshua.hahnjy@xxxxxxxxx>
> ---
>  include/linux/page_counter.h | 16 +++++++
>  mm/page_counter.c            | 90 ++++++++++++++++++++++++++++++++++++
>  2 files changed, 106 insertions(+)
> 
> diff --git a/include/linux/page_counter.h b/include/linux/page_counter.h
> index 89a083f16fbf7..c1fe331f34e7e 100644
> --- a/include/linux/page_counter.h
> +++ b/include/linux/page_counter.h
> @@ -5,8 +5,11 @@
>  #include <linux/atomic.h>
>  #include <linux/cache.h>
>  #include <linux/limits.h>
> +#include <linux/workqueue_types.h>
>  #include <asm/page.h>
>  
> +struct page_counter_stock;
> +
>  struct page_counter {
>  	/*
>  	 * Make sure 'usage' does not share cacheline with any other field in
> @@ -41,6 +44,13 @@ struct page_counter {
>  	unsigned long high;
>  	unsigned long max;
>  	struct page_counter *parent;
> +	struct page_counter_stock __percpu *stock;

On gcc 14.2.1, I get

In file included from mm/page_counter.c:8:
./include/linux/page_counter.h:47:44: error: expected ‘:’, ‘,’, ‘;’, ‘}’ or ‘__attribute__’ before ‘*’ token
   47 |         struct page_counter_stock __percpu *stock;
      |                                            ^

I think you need an appropriate header in this file.