linux-kernel - Re: [PATCH] zram: use notify_free to account all free notifications

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Date:	Fri, 12 Sep 2014 08:21:24 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Jerome Marchand <jmarchan@...hat.com>,
	Nitin Gupta <ngupta@...are.org>,
	Chao Yu <chao2.yu@...sung.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] zram: use notify_free to account all free notifications

Hey Sergey,

Sorry for late review.

On Fri, Sep 05, 2014 at 09:57:09PM +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.
> 
> Use notify_free to account the number of pages freed in zram_free_page(),
> instead of accounting only swap_slot_free_notify() calls (each
> zram_slot_free_notify() call frees one page).
> 
> 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@...il.com>
> ---
>  Documentation/ABI/testing/sysfs-block-zram | 13 ++++++++-----
>  drivers/block/zram/zram_drv.c              |  2 +-
>  2 files changed, 9 insertions(+), 6 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@...are.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..bc20fe1 100644
> --- a/drivers/block/zram/zram_drv.c
> +++ b/drivers/block/zram/zram_drv.c
> @@ -410,6 +410,7 @@ static void zram_free_page(struct zram *zram, size_t index)
>  	atomic64_sub(zram_get_obj_size(meta, index),
>  			&zram->stats.compr_data_size);
>  	atomic64_dec(&zram->stats.pages_stored);
> +	atomic64_inc(&zram->stats.notify_free);

It is wrong. The zram_free_page is called at several places and
they are all not notify free.
Pz, account it in zram_slot_free_notify and zram_bio_discard.

>  
>  	meta->table[index].handle = 0;
>  	zram_set_obj_size(meta, index, 0);
> @@ -938,7 +939,6 @@ static void zram_slot_free_notify(struct block_device *bdev,
>  	bit_spin_lock(ZRAM_ACCESS, &meta->table[index].value);
>  	zram_free_page(zram, index);
>  	bit_spin_unlock(ZRAM_ACCESS, &meta->table[index].value);
> -	atomic64_inc(&zram->stats.notify_free);
>  }
>  
>  static const struct block_device_operations zram_devops = {
> -- 
> 2.1.0.134.gb1ae451
> 
> --
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-- 
Kind regards,
Minchan Kim
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