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Date: Mon, 15 Sep 2014 08:22:49 +0900
From: Minchan Kim <minchan@kernel.org>
To: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        Jerome Marchand <jmarchan@redhat.com>, Nitin Gupta <ngupta@vflare.org>,
        Chao Yu <chao2.yu@samsung.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCHv2] zram: use notify_free to account all free notifications
Message-ID: <20140914232249.GC2160@bbox>
References: <1410580334-8429-1-git-send-email-sergey.senozhatsky@gmail.com>
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Hi Sergey,

On Sat, Sep 13, 2014 at 12:52:14PM +0900, Sergey Senozhatsky wrote:
> notify_free device attribute accounts the number of slot free notifications
> and internally represents the number of zram_free_page() calls. Slot free
> notifications are sent only when device is used as a swap device, hence
> notify_free is used only for swap devices. Since f4659d8e620d08 (zram:
> support REQ_DISCARD) ZRAM handles yet another one free notification (also
> via zram_free_page() call) -- REQ_DISCARD requests, which are sent by a
> filesystem, whenever some data blocks are discarded. However, there is no
> way to know the number of notifications in the latter case.
> 
> Change zram_free_page() to return a bool status, indicating if zs_free()
> has happened. So we can use `notify_free' to account the number of pages
> freed by zram_bio_discard() and zram_slot_free_notify().
> 
> This means that depending on usage scenario `notify_free' represents:
>  a) the number of pages freed because of slot free notifications, which is
>    equal to the number of swap_slot_free_notify() calls, so there is no
>    behaviour change
> 
>  b) the number of pages freed because of REQ_DISCARD notifications
> 
> Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
> ---
>  Documentation/ABI/testing/sysfs-block-zram | 13 ++++++++-----
>  drivers/block/zram/zram_drv.c              | 12 +++++++-----
>  2 files changed, 15 insertions(+), 10 deletions(-)
> 
> diff --git a/Documentation/ABI/testing/sysfs-block-zram b/Documentation/ABI/testing/sysfs-block-zram
> index b13dc99..a6148ea 100644
> --- a/Documentation/ABI/testing/sysfs-block-zram
> +++ b/Documentation/ABI/testing/sysfs-block-zram
> @@ -77,11 +77,14 @@ What:		/sys/block/zram<id>/notify_free
>  Date:		August 2010
>  Contact:	Nitin Gupta <ngupta@vflare.org>
>  Description:
> -		The notify_free file is read-only and specifies the number of
> -		swap slot free notifications received by this device. These
> -		notifications are sent to a swap block device when a swap slot
> -		is freed. This statistic is applicable only when this disk is
> -		being used as a swap disk.
> +		The notify_free file is read-only. Depending on device usage
> +		scenario it may account a) the number of pages freed because
> +		of swap slot free notifications or b) the number of pages freed
> +		because of REQ_DISCARD requests sent by bio. The former ones
> +		are sent to a swap block device when a swap slot is freed, which
> +		implies that this disk is being used as a swap disk. The latter
> +		ones are sent by filesystem mounted with discard option,
> +		whenever some data blocks are getting discarded.
>  
>  What:		/sys/block/zram<id>/zero_pages
>  Date:		August 2010
> diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> index d78b245..03d11d5 100644
> --- a/drivers/block/zram/zram_drv.c
> +++ b/drivers/block/zram/zram_drv.c
> @@ -388,7 +388,7 @@ static void handle_zero_page(struct bio_vec *bvec)
>   * caller should hold this table index entry's bit_spinlock to
>   * indicate this index entry is accessing.
>   */
> -static void zram_free_page(struct zram *zram, size_t index)
> +static bool zram_free_page(struct zram *zram, size_t index)
>  {
>  	struct zram_meta *meta = zram->meta;
>  	unsigned long handle = meta->table[index].handle;
> @@ -402,7 +402,7 @@ static void zram_free_page(struct zram *zram, size_t index)
>  			zram_clear_flag(meta, index, ZRAM_ZERO);
>  			atomic64_dec(&zram->stats.zero_pages);
>  		}
> -		return;
> +		return false;
>  	}
>  
>  	zs_free(meta->mem_pool, handle);
> @@ -413,6 +413,7 @@ static void zram_free_page(struct zram *zram, size_t index)
>  
>  	meta->table[index].handle = 0;
>  	zram_set_obj_size(meta, index, 0);
> +	return true;
>  }

For just stat accounting, adding return value for fast path?
I don't think it's not a important stat at the cost of adding more overhead
in fastpath. If you have a strong reason, I will do that. Otherwise,
please, don't touch fast path and just account it regardless of real freeing.
I should have said before resending.
Sorry for bothering you. :)

>  
>  static int zram_decompress_page(struct zram *zram, char *mem, u32 index)
> @@ -696,7 +697,8 @@ static void zram_bio_discard(struct zram *zram, u32 index,
>  
>  	while (n >= PAGE_SIZE) {
>  		bit_spin_lock(ZRAM_ACCESS, &meta->table[index].value);
> -		zram_free_page(zram, index);
> +		if (zram_free_page(zram, index))
> +			atomic64_inc(&zram->stats.notify_free);
>  		bit_spin_unlock(ZRAM_ACCESS, &meta->table[index].value);
>  		index++;
>  		n -= PAGE_SIZE;
> @@ -936,9 +938,9 @@ static void zram_slot_free_notify(struct block_device *bdev,
>  	meta = zram->meta;
>  
>  	bit_spin_lock(ZRAM_ACCESS, &meta->table[index].value);
> -	zram_free_page(zram, index);
> +	if (zram_free_page(zram, index))
> +		atomic64_inc(&zram->stats.notify_free);
>  	bit_spin_unlock(ZRAM_ACCESS, &meta->table[index].value);
> -	atomic64_inc(&zram->stats.notify_free);
>  }
>  
>  static const struct block_device_operations zram_devops = {
> -- 
> 2.1.0.251.ga182987
> 
> --
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-- 
Kind regards,
Minchan Kim