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

[Sergey Senozhatsky]

Hello,

On (08/26/14 10:43), Chao Yu wrote:
> Hi Sergey,
> > > 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
> 
> As I know, administrator can send discard request by using "swapon -d /dev/zram0"
> So after then, notify_free may show the page number mixed with both result of
> handling REQ_DISCARD and slot free notifications.
> 
> And as I check the code, there is a workqueue "swap_discard_work" inited
> in sys_swapon, so maybe there are REQ_DISCARDs sent from background thread,
> but not filesystem. Is this right?

yes, correct. I didn't know that at that time. as Minchan noted, swap
device may receive both slot free and REQ_DISCARD.

	-ss

> Regards,
> Yu
> 
> > 
> >  b) the number of pages freed because of REQ_DISCARD notifications
> > 
> > Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>
> > ---
> >  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@xxxxxxxxxx>
> >  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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