Re: [PATCH v3 1/6] zbud: use page ref counter for zbud pages

[Seth Jennings <sjenning@linux.vnet.ibm.com>]

On Tue, Oct 08, 2013 at 03:29:35PM +0200, Krzysztof Kozlowski wrote:
> Use page reference counter for zbud pages. The ref counter replaces
> zbud_header.under_reclaim flag and ensures that zbud page won't be freed
> when zbud_free() is called during reclaim. It allows implementation of
> additional reclaim paths.
> 
> The page count is incremented when:
>  - a handle is created and passed to zswap (in zbud_alloc()),
>  - user-supplied eviction callback is called (in zbud_reclaim_page()).
> 
> Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
> Signed-off-by: Tomasz Stanislawski <t.stanislaws@samsung.com>
> Reviewed-by: Bob Liu <bob.liu@oracle.com>

Other than the nit below:

Acked-by: Seth Jennings <sjenning@linux.vnet.ibm.com>

> ---
>  mm/zbud.c |  117 +++++++++++++++++++++++++++++++++----------------------------
>  1 file changed, 64 insertions(+), 53 deletions(-)
> 
> diff --git a/mm/zbud.c b/mm/zbud.c
> index 9451361..7574289 100644
> --- a/mm/zbud.c
> +++ b/mm/zbud.c
> @@ -109,7 +109,6 @@ struct zbud_header {
>  	struct list_head lru;
>  	unsigned int first_chunks;
>  	unsigned int last_chunks;
> -	bool under_reclaim;
>  };
> 
>  /*****************
> @@ -138,16 +137,9 @@ static struct zbud_header *init_zbud_page(struct page *page)
>  	zhdr->last_chunks = 0;
>  	INIT_LIST_HEAD(&zhdr->buddy);
>  	INIT_LIST_HEAD(&zhdr->lru);
> -	zhdr->under_reclaim = 0;
>  	return zhdr;
>  }
> 
> -/* Resets the struct page fields and frees the page */
> -static void free_zbud_page(struct zbud_header *zhdr)
> -{
> -	__free_page(virt_to_page(zhdr));
> -}
> -
>  /*
>   * Encodes the handle of a particular buddy within a zbud page
>   * Pool lock should be held as this function accesses first|last_chunks
> @@ -188,6 +180,31 @@ static int num_free_chunks(struct zbud_header *zhdr)
>  	return NCHUNKS - zhdr->first_chunks - zhdr->last_chunks - 1;
>  }
> 
> +/*
> + * Increases ref count for zbud page.
> + */
> +static void get_zbud_page(struct zbud_header *zhdr)
> +{
> +	get_page(virt_to_page(zhdr));
> +}
> +
> +/*
> + * Decreases ref count for zbud page and frees the page if it reaches 0
> + * (no external references, e.g. handles).
> + *
> + * Returns 1 if page was freed and 0 otherwise.
> + */
> +static int put_zbud_page(struct zbud_header *zhdr)
> +{
> +	struct page *page = virt_to_page(zhdr);
> +	if (put_page_testzero(page)) {
> +		free_hot_cold_page(page, 0);
> +		return 1;
> +	}
> +	return 0;
> +}
> +
> +
>  /*****************
>   * API Functions
>  *****************/
> @@ -273,6 +290,7 @@ int zbud_alloc(struct zbud_pool *pool, int size, gfp_t gfp,
>  				bud = FIRST;
>  			else
>  				bud = LAST;
> +			get_zbud_page(zhdr);
>  			goto found;
>  		}
>  	}
> @@ -284,6 +302,10 @@ int zbud_alloc(struct zbud_pool *pool, int size, gfp_t gfp,
>  		return -ENOMEM;
>  	spin_lock(&pool->lock);
>  	pool->pages_nr++;
> +	/*
> +	 * We will be using zhdr instead of page, so
> +	 * don't increase the page count.
> +	 */

This comment isn't very clear.  I think what you mean to say is that
we already have the page ref'ed for this entry because of the initial
ref count done by alloc_page().

So maybe:

/*
 * Page count is incremented by alloc_page() for the initial
 * reference so no need to call zbud_get_page() here.
 */

Thanks,
Seth

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