linux-kernel - Re: [RFC 3/3] zram: add swap_get

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Date:	Thu, 04 Sep 2014 15:26:14 +0900
From:	Heesub Shin <heesub.shin@...sung.com>
To:	Minchan Kim <minchan@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Hugh Dickins <hughd@...gle.com>, Shaohua Li <shli@...nel.org>,
	Jerome Marchand <jmarchan@...hat.com>,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Dan Streetman <ddstreet@...e.org>,
	Nitin Gupta <ngupta@...are.org>,
	Luigi Semenzato <semenzato@...gle.com>
Subject: Re: [RFC 3/3] zram: add swap_get_free hint

Hello Minchan,

First of all, I agree with the overall purpose of your patch set.

On 09/04/2014 10:39 AM, Minchan Kim wrote:
> This patch implement SWAP_GET_FREE handler in zram so that VM can
> know how many zram has freeable space.
> VM can use it to stop anonymous reclaiming once zram is full.
>
> Signed-off-by: Minchan Kim <minchan@...nel.org>
> ---
>   drivers/block/zram/zram_drv.c | 18 ++++++++++++++++++
>   1 file changed, 18 insertions(+)
>
> diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> index 88661d62e46a..8e22b20aa2db 100644
> --- a/drivers/block/zram/zram_drv.c
> +++ b/drivers/block/zram/zram_drv.c
> @@ -951,6 +951,22 @@ static int zram_slot_free_notify(struct block_device *bdev,
>   	return 0;
>   }
>
> +static int zram_get_free_pages(struct block_device *bdev, long *free)
> +{
> +	struct zram *zram;
> +	struct zram_meta *meta;
> +
> +	zram = bdev->bd_disk->private_data;
> +	meta = zram->meta;
> +
> +	if (!zram->limit_pages)
> +		return 1;
> +
> +	*free = zram->limit_pages - zs_get_total_pages(meta->mem_pool);

Even if 'free' is zero here, there may be free spaces available to store 
more compressed pages into the zs_pool. I mean calculation above is not 
quite accurate and wastes memory, but have no better idea for now.

heesub

> +
> +	return 0;
> +}
> +
>   static int zram_swap_hint(struct block_device *bdev,
>   				unsigned int hint, void *arg)
>   {
> @@ -958,6 +974,8 @@ static int zram_swap_hint(struct block_device *bdev,
>
>   	if (hint == SWAP_SLOT_FREE)
>   		ret = zram_slot_free_notify(bdev, (unsigned long)arg);
> +	else if (hint == SWAP_GET_FREE)
> +		ret = zram_get_free_pages(bdev, arg);
>
>   	return ret;
>   }
>
--
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