Re: [PATCH -next v6 2/2] mm/zswap: delay the initializaton of zswap

Christoph Hellwig
2023-03-23T08:04:53Z

On Wed, Mar 22, 2023 at 06:20:06PM +0800, Liu Shixin wrote:
> Since some users may not use zswap, the zswap_pool is wasted. Save memory
> by delaying the initialization of zswap until enabled.
>
> Signed-off-by: Liu Shixin <liushixin2@huawei.com>
> ---
> mm/zswap.c | 51 +++++++++++++++++++++++++++++++++++++++++----------
> 1 file changed, 41 insertions(+), 10 deletions(-)
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 09fa956920fa..3aed3b26524a 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -81,6 +81,8 @@ static bool zswap_pool_reached_full;
>
> #define ZSWAP_PARAM_UNSET ""
>
> +static int zswap_setup(void);
> +
> /* Enable/disable zswap */
> static bool zswap_enabled = IS_ENABLED(CONFIG_ZSWAP_DEFAULT_ON);
> static int zswap_enabled_param_set(const char *,
> @@ -220,6 +222,9 @@ static bool zswap_init_started;
> /* fatal error during init */
> static bool zswap_init_failed;
>
> +/* used to ensure the integrity of initialization */
> +static DEFINE_MUTEX(zswap_init_lock);
> +
> /* init completed, but couldn't create the initial pool */
> static bool zswap_has_pool;
>
> @@ -272,13 +277,13 @@ static void zswap_update_total_size(void)
> **********************************/
> static struct kmem_cache *zswap_entry_cache;
>
> -static int __init zswap_entry_cache_create(void)
> +static int zswap_entry_cache_create(void)
> {
> zswap_entry_cache = KMEM_CACHE(zswap_entry, 0);
> return zswap_entry_cache == NULL;
> }

Please add a cleanup patch to remove this helper first, it just
massivel confuses the reader.

> -static void __init zswap_entry_cache_destroy(void)
> +static void zswap_entry_cache_destroy(void)
> {
> kmem_cache_destroy(zswap_entry_cache);
> }

Same here.

> @@ -663,7 +668,7 @@ static struct zswap_pool *zswap_pool_create(char *type, char *compressor)
> return NULL;
> }
>
> -static __init struct zswap_pool *__zswap_pool_create_fallback(void)
> +static struct zswap_pool *__zswap_pool_create_fallback(void)
> {
> bool has_comp, has_zpool;
>
> @@ -784,8 +789,15 @@ static int __zswap_param_set(const char *val, const struct kernel_param *kp,
> /* if this is load-time (pre-init) param setting,
> * don't create a pool; that's done during init.
> */
> - if (!zswap_init_started)
> - return param_set_charp(s, kp);
> + if (!zswap_init_started) {
> + mutex_lock(&zswap_init_lock);
> + if (!zswap_init_started) {
> + ret = param_set_charp(s, kp);
> + mutex_unlock(&zswap_init_lock);
> + return ret;
> + }
> + mutex_unlock(&zswap_init_lock);
> + }

Just take the lock around the whole function. No need to micro-optimize
setting a kernel paramter.

> @@ -884,6 +896,15 @@ static int zswap_enabled_param_set(const char *val,
> if (res == *(bool *)kp->arg)
> return 0;
>
> + if (!zswap_init_started && (system_state == SYSTEM_RUNNING)) {

No need for the inner braces. But directly looking at
SYSTEM_RUNNING, especially without a comment is a bit of a mess.
Is there any better way to deal with this?

Also the zswap_init_started variable name has always been a bit
confusing. If everything around it takes zswap_init_lock now,
it can be replaced with a check for successful zswap initialization
as all the initializtion is covered by the lock. That would really
help to clean up the code.

> +static int zswap_debugfs_init(void)
> {
> if (!debugfs_initialized())
> return -ENODEV;
> @@ -1482,7 +1503,7 @@ static int __init zswap_debugfs_init(void)
> return 0;
> }
> #else
> -static int __init zswap_debugfs_init(void)
> +static int zswap_debugfs_init(void)

Is there any reason to not just always initialize debugfs and
only defer the expensive allocations?