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

[Sergey Senozhatsky <sergey.senozhatsky@gmail.com>]

Hello Minchan,

On (09/12/14 08:21), Minchan Kim wrote:
> 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@gmail.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@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..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.

wow, why did I do that... good catch!
will resend shortly.
sorry.

	-ss

> >  
> >  	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
>