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Date:   Mon, 11 Apr 2022 19:16:21 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Yu Zhao <yuzhao@google.com>
Cc:     Stephen Rothwell <sfr@rothwell.id.au>, linux-mm@kvack.org,
        Andi Kleen <ak@linux.intel.com>,
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Subject: Re: [PATCH v10 08/14] mm: multi-gen LRU: support page table walks
Message-Id: <20220411191621.0378467ad99ebc822d5ad005@linux-foundation.org>
In-Reply-To: <20220407031525.2368067-9-yuzhao@google.com>
References: <20220407031525.2368067-1-yuzhao@google.com>
        <20220407031525.2368067-9-yuzhao@google.com>
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On Wed,  6 Apr 2022 21:15:20 -0600 Yu Zhao <yuzhao@google.com> wrote:

>
> ...
>
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -6083,6 +6083,29 @@ static void mem_cgroup_move_task(void)
>  }
>  #endif
>  
> +#ifdef CONFIG_LRU_GEN
> +static void mem_cgroup_attach(struct cgroup_taskset *tset)
> +{
> +	struct cgroup_subsys_state *css;
> +	struct task_struct *task = NULL;
> +
> +	cgroup_taskset_for_each_leader(task, css, tset)
> +		break;

Does this actually do anything?

> +	if (!task)
> +		return;
> +
> +	task_lock(task);
> +	if (task->mm && task->mm->owner == task)
> +		lru_gen_migrate_mm(task->mm);
> +	task_unlock(task);
> +}
>  
> ...
>
> +static void update_batch_size(struct lru_gen_mm_walk *walk, struct folio *folio,
> +			      int old_gen, int new_gen)
> +{
> +	int type = folio_is_file_lru(folio);
> +	int zone = folio_zonenum(folio);
> +	int delta = folio_nr_pages(folio);
> +
> +	VM_BUG_ON(old_gen >= MAX_NR_GENS);
> +	VM_BUG_ON(new_gen >= MAX_NR_GENS);

General rule: don't add new BUG_ONs, because they crash the kenrel. 
It's better to use WARN_ON or WARN_ON_ONCE then try to figure out a way
to keep the kernel limping along.  At least so the poor user can gather logs.

> +	walk->batched++;
> +
> +	walk->nr_pages[old_gen][type][zone] -= delta;
> +	walk->nr_pages[new_gen][type][zone] += delta;
> +}
> +