Subject: Re: [PATCH 12/13] mm: memcontrol: account socket memory in unified hierarchy memory controller

From: Vladimir Davydov <[email-masked]>
Date: Mon, 30 Nov 2015 13:54:21 +0300

On Tue, Nov 24, 2015 at 04:58:44PM -0500, Johannes Weiner wrote:
...
> @@ -5520,15 +5557,30 @@ void sock_release_memcg(struct sock *sk)
>   */
>  bool mem_cgroup_charge_skmem(struct mem_cgroup *memcg, unsigned int nr_pages)
>  {
> -	struct page_counter *counter;
> +	gfp_t gfp_mask = GFP_KERNEL;
>  
> -	if (page_counter_try_charge(&memcg->tcp_mem.memory_allocated,
> -				    nr_pages, &counter)) {
> -		memcg->tcp_mem.memory_pressure = 0;
> -		return true;
> +#ifdef CONFIG_MEMCG_KMEM
> +	if (!cgroup_subsys_on_dfl(memory_cgrp_subsys)) {
> +		struct page_counter *counter;
> +
> +		if (page_counter_try_charge(&memcg->tcp_mem.memory_allocated,
> +					    nr_pages, &counter)) {
> +			memcg->tcp_mem.memory_pressure = 0;
> +			return true;
> +		}
> +		page_counter_charge(&memcg->tcp_mem.memory_allocated, nr_pages);
> +		memcg->tcp_mem.memory_pressure = 1;
> +		return false;
>  	}
> -	page_counter_charge(&memcg->tcp_mem.memory_allocated, nr_pages);
> -	memcg->tcp_mem.memory_pressure = 1;
> +#endif
> +	/* Don't block in the packet receive path */
> +	if (in_softirq())
> +		gfp_mask = GFP_NOWAIT;
> +
> +	if (try_charge(memcg, gfp_mask, nr_pages) == 0)
> +		return true;
> +
> +	try_charge(memcg, gfp_mask|__GFP_NOFAIL, nr_pages);

We won't trigger high reclaim if we get here, because try_charge does
not check high threshold if failing or forcing charge. I think this
should be fixed regardless of this patch. The fix is attached below.

Also, I don't like calling try_charge twice: the second time will go
through all the try_charge steps for nothing. What about checking
page_counter value after calling try_charge instead:

	try_charge(memcg, gfp_mask|__GFP_NOFAIL, nr_pages);
	return page_counter_read(&memcg->memory) <= memcg->memory.limit;

or adding an out parameter to try_charge that would inform us if charge
was forced?

>  	return false;
>  }
>  
> @@ -5539,10 +5591,32 @@ bool mem_cgroup_charge_skmem(struct mem_cgroup *memcg, unsigned int nr_pages)
>   */
>  void mem_cgroup_uncharge_skmem(struct mem_cgroup *memcg, unsigned int nr_pages)
>  {
> -	page_counter_uncharge(&memcg->tcp_mem.memory_allocated, nr_pages);
> +#ifdef CONFIG_MEMCG_KMEM
> +	if (!cgroup_subsys_on_dfl(memory_cgrp_subsys)) {
> +		page_counter_uncharge(&memcg->tcp_mem.memory_allocated,
> +				      nr_pages);
> +		return;
> +	}
> +#endif
> +	page_counter_uncharge(&memcg->memory, nr_pages);
> +	css_put_many(&memcg->css, nr_pages);

cancel_charge(memcg, nr_pages);

?

---
From: Vladimir Davydov <[email-masked]>
Subject: [PATCH] memcg: check high threshold if forcing allocation

try_charge() does not result in checking high threshold if it forces
charge. This is incorrect, because we could have failed to reclaim
memory due to the current context, so we do need to check high threshold
and try to compensate for the excess once we are in the safe context.

Signed-off-by: Vladimir Davydov <[email-masked]>

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 79a29d564bff..e922965b572b 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2112,13 +2112,14 @@ static int try_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
 		page_counter_charge(&memcg->memsw, nr_pages);
 	css_get_many(&memcg->css, nr_pages);
 
-	return 0;
+	goto check_high;
 
 done_restock:
 	css_get_many(&memcg->css, batch);
 	if (batch > nr_pages)
 		refill_stock(memcg, batch - nr_pages);
 
+check_high:
 	/*
 	 * If the hierarchy is above the normal consumption range, schedule
 	 * reclaim on returning to userland.  We can perform reclaim here