Re: [PATCH] mm: use unique zsmalloc caches names

[Andrew Morton <akpm@linux-foundation.org>]

On Thu,  5 Sep 2024 15:47:23 +0900 Sergey Senozhatsky <senozhatsky@chromium.org> wrote:

> Each zsmalloc pool maintains several named kmem-caches for
> zs_handle-s and  zspage-s.  On a system with multiple zsmalloc
> pools and CONFIG_DEBUG_VM this triggers kmem_cache_sanity_check():
> 
>   kmem_cache of name 'zspage' already exists
>   WARNING: at mm/slab_common.c:108 do_kmem_cache_create_usercopy+0xb5/0x310
>   ...
> 
>   kmem_cache of name 'zs_handle' already exists
>   WARNING: at mm/slab_common.c:108 do_kmem_cache_create_usercopy+0xb5/0x310
>   ...

This is old code.  Did something recently change to trigger this warning?

> We provide zram device name when init its zsmalloc pool, so we can
> use that same name for zsmalloc caches and, hence, create unique
> names that can easily be linked to zram device that has created
> them.
> 
> So instead of having this
> 
> cat /proc/slabinfo
> slabinfo - version: 2.1
> zspage                46     46    ...
> zs_handle            128    128    ...
> zspage             34270  34270    ...
> zs_handle          34816  34816    ...
> zspage                 0      0    ...
> zs_handle              0      0    ...
> 
> We now have this
> 
> cat /proc/slabinfo
> slabinfo - version: 2.1
> zspage-zram2          46     46    ...
> zs_handle-zram2      128    128    ...
> zspage-zram0       34270  34270    ...
> zs_handle-zram0    34816  34816    ...
> zspage-zram1           0      0    ...
> zs_handle-zram1        0      0    ...
> 
> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c
> @@ -293,13 +293,17 @@ static void SetZsPageMovable(struct zs_pool *pool, struct zspage *zspage) {}
>  
>  static int create_cache(struct zs_pool *pool)
>  {
> -	pool->handle_cachep = kmem_cache_create("zs_handle", ZS_HANDLE_SIZE,
> -					0, 0, NULL);
> +	char name[32];
> +
> +	snprintf(name, sizeof(name), "zs_handle-%s", pool->name);

Always scary seeing code making such assumptions about it arguments in
this fashion.  Can we use kasprintf() and sleep well at night?


> +	pool->handle_cachep = kmem_cache_create(name, ZS_HANDLE_SIZE,
> +						0, 0, NULL);
>  	if (!pool->handle_cachep)
>  		return 1;
>  
> -	pool->zspage_cachep = kmem_cache_create("zspage", sizeof(struct zspage),
> -					0, 0, NULL);
> +	snprintf(name, sizeof(name), "zspage-%s", pool->name);
> +	pool->zspage_cachep = kmem_cache_create(name, sizeof(struct zspage),
> +						0, 0, NULL);
>  	if (!pool->zspage_cachep) {
>  		kmem_cache_destroy(pool->handle_cachep);
>  		pool->handle_cachep = NULL;

I guess we want to backport this into earlier kernels?  If so, what
would be a suitable Fixes:?