linux-kernel - Re: [PATCH] zram: add num_discards for discarded pages stat

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Date:	Thu, 21 Aug 2014 22:05:04 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:	Chao Yu <chao2.yu@...sung.com>
Cc:	'Minchan Kim' <minchan@...nel.org>,
	'Sergey Senozhatsky' <sergey.senozhatsky@...il.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	ngupta@...are.org, 'Jerome Marchand' <jmarchan@...hat.com>,
	'Andrew Morton' <akpm@...ux-foundation.org>
Subject: Re: [PATCH] zram: add num_discards for discarded pages stat

On (08/21/14 17:09), Chao Yu wrote:
[cut]
> > 
> > I hope I'm not discouraging. :)
> 
> Nope, please let me try again, :)
> 
> Since we have supported handling discard request in this commit
> f4659d8e620d08bd1a84a8aec5d2f5294a242764 (zram: support REQ_DISCARD), zram got
> one more chance to free unused memory whenever received discard request. But
> without stating for discard request, there is no method for user to know whether
> discard request has been handled by zram or how many blocks were discarded by
> zram when user wants to know the effect of discard.
> 
> In this patch, we add num_discards to stat discarded pages, and export it to
> sysfs for users.
> 

In other words, here is my proposal:

-----8<-----8<-----

Subject: [PATCH] zram: use notify_free to account all free notifications

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 70ec992..73ed400 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 swap slot free
+		notifications or b) the number 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 d00831c..c2e7127 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -344,6 +344,7 @@ 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);
 
 	meta->table[index].handle = 0;
 	zram_set_obj_size(meta, index, 0);
@@ -843,7 +844,6 @@ 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 = {
-- 
2.1.0.233.g9eef2c8

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