Re: [PATCH 2/8] mm: frontswap: lazy initialization to allow tmem backends to build/run as modules

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

On Wed, 14 Nov 2012 13:57:06 -0500
Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> wrote:

> From: Dan Magenheimer <dan.magenheimer@oracle.com>
> 
> With the goal of allowing tmem backends (zcache, ramster, Xen tmem) to be
> built/loaded as modules rather than built-in and enabled by a boot parameter,
> this patch provides "lazy initialization", allowing backends to register to
> frontswap even after swapon was run. Before a backend registers all calls
> to init are recorded and the creation of tmem_pools delayed until a backend
> registers or until a frontswap put is attempted.
> 
>
> ...
>
> --- a/mm/frontswap.c
> +++ b/mm/frontswap.c
> @@ -80,6 +80,18 @@ static inline void inc_frontswap_succ_stores(void) { }
>  static inline void inc_frontswap_failed_stores(void) { }
>  static inline void inc_frontswap_invalidates(void) { }
>  #endif
> +
> +/*
> + * When no backend is registered all calls to init are registered and

What is "init"?  Spell it out fully, please.

> + * remembered but fail to create tmem_pools. When a backend registers with
> + * frontswap the previous calls to init are executed to create tmem_pools
> + * and set the respective poolids.

Again, seems really hacky.  Why can't we just change callers so they
call things in the correct order?

> + * While no backend is registered all "puts", "gets" and "flushes" are
> + * ignored or fail.
> + */
> +static DECLARE_BITMAP(need_init, MAX_SWAPFILES);
> +static bool backend_registered __read_mostly;
> +
>  /*
>   * Register operations for frontswap, returning previous thus allowing
>   * detection of multiple backends and possible nesting.
> @@ -87,9 +99,19 @@ static inline void inc_frontswap_invalidates(void) { }
>  struct frontswap_ops frontswap_register_ops(struct frontswap_ops *ops)
>  {
>  	struct frontswap_ops old = frontswap_ops;
> +	int i;
>  
>  	frontswap_ops = *ops;
>  	frontswap_enabled = true;
> +
> +	for (i = 0; i < MAX_SWAPFILES; i++) {
> +		if (test_and_clear_bit(i, need_init))

ooh, that wasn't racy ;)

> +			(*frontswap_ops.init)(i);
> +	}
> +	/* We MUST have backend_registered called _after_ the frontswap_init's
> + 	 * have been called. Otherwise __frontswap_store might fail. */

Comment makes no sense - backend_registered is not a function.

Also, let's lay the comments out conventionally please:

	/*
	 * We MUST have backend_registered called _after_ the frontswap_init's
	 * have been called. Otherwise __frontswap_store might fail.
	 */


> +	barrier();
> +	backend_registered = true;
>  	return old;
>  }
>  EXPORT_SYMBOL(frontswap_register_ops);
>
> ...
>
> @@ -226,12 +266,15 @@ void __frontswap_invalidate_area(unsigned type)
>  {
>  	struct swap_info_struct *sis = swap_info[type];
>  
> -	BUG_ON(sis == NULL);
> -	if (sis->frontswap_map == NULL)
> -		return;
> -	frontswap_ops.invalidate_area(type);
> -	atomic_set(&sis->frontswap_pages, 0);
> -	memset(sis->frontswap_map, 0, sis->max / sizeof(long));
> +	if (backend_registered) {
> +		BUG_ON(sis == NULL);
> +		if (sis->frontswap_map == NULL)
> +			return;
> +		(*frontswap_ops.invalidate_area)(type);
> +		atomic_set(&sis->frontswap_pages, 0);
> +		memset(sis->frontswap_map, 0, sis->max / sizeof(long));
> +	}
> +	clear_bit(type, need_init);
>  }
>  EXPORT_SYMBOL(__frontswap_invalidate_area);
>  
> @@ -364,6 +407,9 @@ static int __init init_frontswap(void)
>  	debugfs_create_u64("invalidates", S_IRUGO,
>  				root, &frontswap_invalidates);
>  #endif
> +	bitmap_zero(need_init, MAX_SWAPFILES);

unneeded?

> +	frontswap_enabled = 1;
>  	return 0;
>  }
>
> ...
>

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