linux-kernel - Re: [PATCH v5 08/12] zsmalloc: introduce zspage structure

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Date:	Mon, 16 May 2016 12:09:41 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:	Minchan Kim <minchan@...nel.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [PATCH v5 08/12] zsmalloc: introduce zspage structure

On (05/09/16 11:20), Minchan Kim wrote:
> We have squeezed meta data of zspage into first page's descriptor.
> So, to get meta data from subpage, we should get first page first
> of all. But it makes trouble to implment page migration feature
> of zsmalloc because any place where to get first page from subpage
> can be raced with first page migration. IOW, first page it got
> could be stale. For preventing it, I have tried several approahces
> but it made code complicated so finally, I concluded to separate
> metadata from first page. Of course, it consumes more memory. IOW,
> 16bytes per zspage on 32bit at the moment. It means we lost 1%
> at *worst case*(40B/4096B) which is not bad I think at the cost of
> maintenance.
> 
> Cc: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
> Signed-off-by: Minchan Kim <minchan@...nel.org>
[..]
> @@ -153,8 +138,6 @@
>  enum fullness_group {
>  	ZS_ALMOST_FULL,
>  	ZS_ALMOST_EMPTY,
> -	_ZS_NR_FULLNESS_GROUPS,
> -
>  	ZS_EMPTY,
>  	ZS_FULL
>  };
> @@ -203,7 +186,7 @@ static const int fullness_threshold_frac = 4;
>  
>  struct size_class {
>  	spinlock_t lock;
> -	struct page *fullness_list[_ZS_NR_FULLNESS_GROUPS];
> +	struct list_head fullness_list[2];

seems that it also has some cleaup bits in it.

[..]
> -static int create_handle_cache(struct zs_pool *pool)
> +static int create_cache(struct zs_pool *pool)
>  {
>  	pool->handle_cachep = kmem_cache_create("zs_handle", ZS_HANDLE_SIZE,
>  					0, 0, NULL);
> -	return pool->handle_cachep ? 0 : 1;
> +	if (!pool->handle_cachep)
> +		return 1;
> +
> +	pool->zspage_cachep = kmem_cache_create("zspage", sizeof(struct zspage),
> +					0, 0, NULL);
> +	if (!pool->zspage_cachep) {
> +		kmem_cache_destroy(pool->handle_cachep);
		^^^^^

do you need to NULL a pool->handle_cachep here?

zs_create_pool()
	if (create_cache() == 1) {
			pool->zspage_cachep NULL
			pool->handle_cachep !NULL   already freed -> kmem_cache_destroy()
			return 1;
		goto err
	}
err:
	zs_destroy_pool()
		destroy_cache() {
			kmem_cache_destroy(pool->handle_cachep);  !NULL and freed
			kmem_cache_destroy(pool->zspage_cachep);  NULL ok
		}


can we also switch create_cache() to errnos? I just like a bit
better
		return -ENOMEM;
	else
		return 0;

than

		return 1;
	else
		return 0;


> @@ -997,44 +951,38 @@ static void init_zspage(struct size_class *class, struct page *first_page)
>  		off %= PAGE_SIZE;
>  	}
>  
> -	set_freeobj(first_page, (unsigned long)location_to_obj(first_page, 0));
> +	set_freeobj(zspage,
> +		(unsigned long)location_to_obj(zspage->first_page, 0));

	static unsigned long location_to_obj()

it's already returning "(unsigned long)", so here and in several other places
this cast can be dropped.

[..]
> +static struct zspage *isolate_zspage(struct size_class *class, bool source)
>  {
> +	struct zspage *zspage;
> +	enum fullness_group fg[2] = {ZS_ALMOST_EMPTY, ZS_ALMOST_FULL};
> +	if (!source) {
> +		fg[0] = ZS_ALMOST_FULL;
> +		fg[1] = ZS_ALMOST_EMPTY;
> +	}
> +
> +	for (i = 0; i < 2; i++) {

sorry, why not "for (i = ZS_ALMOST_EMPTY; i <= ZS_ALMOST_FULL ..." ?

> +		zspage = list_first_entry_or_null(&class->fullness_list[fg[i]],
> +							struct zspage, list);
> +		if (zspage) {
> +			remove_zspage(class, zspage, fg[i]);
> +			return zspage;
>  		}
>  	}
>  
> -	return page;
> +	return zspage;
>  }

	-ss