linux-kernel - Re: [PATCH v3 1/2] mm/zswap: global lru and shrinker shared by all zswap

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Date: Mon, 19 Feb 2024 17:28:04 -0800
From: Nhat Pham <nphamcs@...il.com>
To: Chengming Zhou <zhouchengming@...edance.com>
Cc: Johannes Weiner <hannes@...xchg.org>, Yosry Ahmed <yosryahmed@...gle.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/2] mm/zswap: global lru and shrinker shared by all zswap_pools

On Fri, Feb 16, 2024 at 12:55 AM Chengming Zhou
<zhouchengming@...edance.com> wrote:
>
> Dynamic zswap_pool creation may create/reuse to have multiple
> zswap_pools in a list, only the first will be current used.
>
> Each zswap_pool has its own lru and shrinker, which is not
> necessary and has its problem:
>
> 1. When memory has pressure, all shrinker of zswap_pools will
>    try to shrink its own lru, there is no order between them.
>
> 2. When zswap limit hit, only the last zswap_pool's shrink_work
>    will try to shrink its lru list. The rationale here was to
>    try and empty the old pool first so that we can completely
>    drop it. However, since we only support exclusive loads now,
>    the LRU ordering should be entirely decided by the order of
>    stores, so the oldest entries on the LRU will naturally be
>    from the oldest pool.
>
> Anyway, having a global lru and shrinker shared by all zswap_pools
> is better and efficient.
>
> Acked-by: Yosry Ahmed <yosryahmed@...gle.com>
> Signed-off-by: Chengming Zhou <zhouchengming@...edance.com>
> ---
>  mm/zswap.c | 171 ++++++++++++++++++++++++-------------------------------------
>  1 file changed, 66 insertions(+), 105 deletions(-)
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 62fe307521c9..d275eb523fc4 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -176,14 +176,19 @@ struct zswap_pool {
>         struct kref kref;
>         struct list_head list;
>         struct work_struct release_work;
> -       struct work_struct shrink_work;
>         struct hlist_node node;
>         char tfm_name[CRYPTO_MAX_ALG_NAME];
> +};
> +
> +static struct {
>         struct list_lru list_lru;
> -       struct mem_cgroup *next_shrink;
> -       struct shrinker *shrinker;
>         atomic_t nr_stored;
> -};
> +       struct shrinker *shrinker;
> +       struct work_struct shrink_work;
> +       struct mem_cgroup *next_shrink;
> +       /* The lock protects next_shrink. */
> +       spinlock_t shrink_lock;
> +} zswap;

nit: Is there a reason why we're putting these in a struct instead of
just a bunch of static variables (perhaps prefixed with zswap?)