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Date: Mon, 15 Sep 2014 08:22:49 +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: [PATCHv2] zram: use notify_free to account all free notifications
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@...il.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@...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..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
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