[PATCH v3] zram: add num_discards for discarded pages stat

[PATCH v3] zram: add num_discards for discarded pages stat

[Chao Yu]

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.

* From v1
 * Update zram document to show num_discards in statistics list.

* From v2
 * Update description of this patch with clear goal.

Signed-off-by: Chao Yu <chao2.yu@samsung.com>
---
 Documentation/ABI/testing/sysfs-block-zram | 10 ++++++++++
 Documentation/blockdev/zram.txt            |  1 +
 drivers/block/zram/zram_drv.c              |  3 +++
 drivers/block/zram/zram_drv.h              |  1 +
 4 files changed, 15 insertions(+)

diff --git a/Documentation/ABI/testing/sysfs-block-zram b/Documentation/ABI/testing/sysfs-block-zram
index 70ec992..fa8936e 100644
--- a/Documentation/ABI/testing/sysfs-block-zram
+++ b/Documentation/ABI/testing/sysfs-block-zram
@@ -57,6 +57,16 @@ Description:
 		The failed_writes file is read-only and specifies the number of
 		failed writes happened on this device.
 
+
+What:		/sys/block/zram<id>/num_discards
+Date:		August 2014
+Contact:	Chao Yu <chao2.yu@samsung.com>
+Description:
+		The num_discards file is read-only and specifies the number of
+		physical blocks which are discarded by this device. These blocks
+		are included in discard request which is sended by filesystem as
+		the blocks are no longer used.
+
 What:		/sys/block/zram<id>/max_comp_streams
 Date:		February 2014
 Contact:	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
diff --git a/Documentation/blockdev/zram.txt b/Documentation/blockdev/zram.txt
index 0595c3f..e50e18b 100644
--- a/Documentation/blockdev/zram.txt
+++ b/Documentation/blockdev/zram.txt
@@ -89,6 +89,7 @@ size of the disk when not in use so a huge zram is wasteful.
 		num_writes
 		failed_reads
 		failed_writes
+		num_discards
 		invalid_io
 		notify_free
 		zero_pages
diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index d00831c..904e7a5 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -606,6 +606,7 @@ static void zram_bio_discard(struct zram *zram, u32 index,
 		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.num_discards);
 		index++;
 		n -= PAGE_SIZE;
 	}
@@ -866,6 +867,7 @@ ZRAM_ATTR_RO(num_reads);
 ZRAM_ATTR_RO(num_writes);
 ZRAM_ATTR_RO(failed_reads);
 ZRAM_ATTR_RO(failed_writes);
+ZRAM_ATTR_RO(num_discards);
 ZRAM_ATTR_RO(invalid_io);
 ZRAM_ATTR_RO(notify_free);
 ZRAM_ATTR_RO(zero_pages);
@@ -879,6 +881,7 @@ static struct attribute *zram_disk_attrs[] = {
 	&dev_attr_num_writes.attr,
 	&dev_attr_failed_reads.attr,
 	&dev_attr_failed_writes.attr,
+	&dev_attr_num_discards.attr,
 	&dev_attr_invalid_io.attr,
 	&dev_attr_notify_free.attr,
 	&dev_attr_zero_pages.attr,
diff --git a/drivers/block/zram/zram_drv.h b/drivers/block/zram/zram_drv.h
index e0f725c..2994aaf 100644
--- a/drivers/block/zram/zram_drv.h
+++ b/drivers/block/zram/zram_drv.h
@@ -86,6 +86,7 @@ struct zram_stats {
 	atomic64_t num_writes;	/* --do-- */
 	atomic64_t failed_reads;	/* can happen when memory is too low */
 	atomic64_t failed_writes;	/* can happen when memory is too low */
+	atomic64_t num_discards;	/* no. of discarded pages */
 	atomic64_t invalid_io;	/* non-page-aligned I/O requests */
 	atomic64_t notify_free;	/* no. of swap slot free notifications */
 	atomic64_t zero_pages;		/* no. of zero filled pages */
-- 
2.0.1.474.g72c7794

Re: [PATCH v3] zram: add num_discards for discarded pages stat

[Minchan Kim]

Hello Chao,

On Fri, Aug 22, 2014 at 04:21:01PM +0800, Chao Yu wrote:
> 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.

My concern is that how much we are able to know the effect of discard
exactly with your patch.

The issue I can think of is zram-swap discard.
Now, zram handles notification from VM to free duplicated copy between
VM-owned memory and zRAM-owned's one so discarding for zram-swap might
be pointless overhead but your stat indicates lots of free page discarded
without real freeing so that user might think "We should keep enable
swap discard for zRAM because the stat indicates it's really good".

In summary, wouldn't it better to have two?

num_discards,
num_failed_discards?

For it, we should modify zram_free_page has return value.
What do other guys think?

> 
> In this patch, we add num_discards to stat discarded pages, and export it to
> sysfs for users.
> 
> * From v1
>  * Update zram document to show num_discards in statistics list.
> 
> * From v2
>  * Update description of this patch with clear goal.
> 
> Signed-off-by: Chao Yu <chao2.yu@samsung.com>
> ---
>  Documentation/ABI/testing/sysfs-block-zram | 10 ++++++++++
>  Documentation/blockdev/zram.txt            |  1 +
>  drivers/block/zram/zram_drv.c              |  3 +++
>  drivers/block/zram/zram_drv.h              |  1 +
>  4 files changed, 15 insertions(+)
> 
> diff --git a/Documentation/ABI/testing/sysfs-block-zram b/Documentation/ABI/testing/sysfs-block-zram
> index 70ec992..fa8936e 100644
> --- a/Documentation/ABI/testing/sysfs-block-zram
> +++ b/Documentation/ABI/testing/sysfs-block-zram
> @@ -57,6 +57,16 @@ Description:
>  		The failed_writes file is read-only and specifies the number of
>  		failed writes happened on this device.
>  
> +
> +What:		/sys/block/zram<id>/num_discards
> +Date:		August 2014
> +Contact:	Chao Yu <chao2.yu@samsung.com>
> +Description:
> +		The num_discards file is read-only and specifies the number of
> +		physical blocks which are discarded by this device. These blocks
> +		are included in discard request which is sended by filesystem as
> +		the blocks are no longer used.
> +
>  What:		/sys/block/zram<id>/max_comp_streams
>  Date:		February 2014
>  Contact:	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
> diff --git a/Documentation/blockdev/zram.txt b/Documentation/blockdev/zram.txt
> index 0595c3f..e50e18b 100644
> --- a/Documentation/blockdev/zram.txt
> +++ b/Documentation/blockdev/zram.txt
> @@ -89,6 +89,7 @@ size of the disk when not in use so a huge zram is wasteful.
>  		num_writes
>  		failed_reads
>  		failed_writes
> +		num_discards
>  		invalid_io
>  		notify_free
>  		zero_pages
> diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> index d00831c..904e7a5 100644
> --- a/drivers/block/zram/zram_drv.c
> +++ b/drivers/block/zram/zram_drv.c
> @@ -606,6 +606,7 @@ static void zram_bio_discard(struct zram *zram, u32 index,
>  		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.num_discards);
>  		index++;
>  		n -= PAGE_SIZE;
>  	}
> @@ -866,6 +867,7 @@ ZRAM_ATTR_RO(num_reads);
>  ZRAM_ATTR_RO(num_writes);
>  ZRAM_ATTR_RO(failed_reads);
>  ZRAM_ATTR_RO(failed_writes);
> +ZRAM_ATTR_RO(num_discards);
>  ZRAM_ATTR_RO(invalid_io);
>  ZRAM_ATTR_RO(notify_free);
>  ZRAM_ATTR_RO(zero_pages);
> @@ -879,6 +881,7 @@ static struct attribute *zram_disk_attrs[] = {
>  	&dev_attr_num_writes.attr,
>  	&dev_attr_failed_reads.attr,
>  	&dev_attr_failed_writes.attr,
> +	&dev_attr_num_discards.attr,
>  	&dev_attr_invalid_io.attr,
>  	&dev_attr_notify_free.attr,
>  	&dev_attr_zero_pages.attr,
> diff --git a/drivers/block/zram/zram_drv.h b/drivers/block/zram/zram_drv.h
> index e0f725c..2994aaf 100644
> --- a/drivers/block/zram/zram_drv.h
> +++ b/drivers/block/zram/zram_drv.h
> @@ -86,6 +86,7 @@ struct zram_stats {
>  	atomic64_t num_writes;	/* --do-- */
>  	atomic64_t failed_reads;	/* can happen when memory is too low */
>  	atomic64_t failed_writes;	/* can happen when memory is too low */
> +	atomic64_t num_discards;	/* no. of discarded pages */
>  	atomic64_t invalid_io;	/* non-page-aligned I/O requests */
>  	atomic64_t notify_free;	/* no. of swap slot free notifications */
>  	atomic64_t zero_pages;		/* no. of zero filled pages */
> -- 
> 2.0.1.474.g72c7794
> 
> 
> --
> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> the body to majordomo@kvack.org.  For more info on Linux MM,
> see: http://www.linux-mm.org/ .
> Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

-- 
Kind regards,
Minchan Kim

Re: [PATCH v3] zram: add num_discards for discarded pages stat

[Sergey Senozhatsky]

Hello,

On (08/25/14 09:36), Minchan Kim wrote:
> Hello Chao,
> 
> On Fri, Aug 22, 2014 at 04:21:01PM +0800, Chao Yu wrote:
> > 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.
> 
> My concern is that how much we are able to know the effect of discard
> exactly with your patch.
> 
> The issue I can think of is zram-swap discard.
> Now, zram handles notification from VM to free duplicated copy between
> VM-owned memory and zRAM-owned's one so discarding for zram-swap might
> be pointless overhead but your stat indicates lots of free page discarded
> without real freeing 

this is why I've moved stats accounting to the place where actual
zs_free() happens. and, frankly, I still would like to see the number
of zs_free() calls, rather than the number of slot free notifications
and REQ_DISCARD (or separately), because they all end up calling
zs_free(). iow, despite the call path, from the user point of view
they are just zs_free() -- the number of pages that's been freed by
the 3rd party and we had have to deal with that.

> so that user might think "We should keep enable
> swap discard for zRAM because the stat indicates it's really good".
> 
> In summary, wouldn't it better to have two?
> 
> num_discards,
> num_failed_discards?

do we actully need this? the only value I can think of (perhaps I'm
missing something) is that we can make sure that we need to support
both slot free and REQ_DISCARDS, or we can leave only REQ_DISCARDS.
is there anything else?

	-ss

> For it, we should modify zram_free_page has return value.
> What do other guys think?
> 
> > 
> > In this patch, we add num_discards to stat discarded pages, and export it to
> > sysfs for users.
> > 
> > * From v1
> >  * Update zram document to show num_discards in statistics list.
> > 
> > * From v2
> >  * Update description of this patch with clear goal.
> > 
> > Signed-off-by: Chao Yu <chao2.yu@samsung.com>
> > ---
> >  Documentation/ABI/testing/sysfs-block-zram | 10 ++++++++++
> >  Documentation/blockdev/zram.txt            |  1 +
> >  drivers/block/zram/zram_drv.c              |  3 +++
> >  drivers/block/zram/zram_drv.h              |  1 +
> >  4 files changed, 15 insertions(+)
> > 
> > diff --git a/Documentation/ABI/testing/sysfs-block-zram b/Documentation/ABI/testing/sysfs-block-zram
> > index 70ec992..fa8936e 100644
> > --- a/Documentation/ABI/testing/sysfs-block-zram
> > +++ b/Documentation/ABI/testing/sysfs-block-zram
> > @@ -57,6 +57,16 @@ Description:
> >  		The failed_writes file is read-only and specifies the number of
> >  		failed writes happened on this device.
> >  
> > +
> > +What:		/sys/block/zram<id>/num_discards
> > +Date:		August 2014
> > +Contact:	Chao Yu <chao2.yu@samsung.com>
> > +Description:
> > +		The num_discards file is read-only and specifies the number of
> > +		physical blocks which are discarded by this device. These blocks
> > +		are included in discard request which is sended by filesystem as
> > +		the blocks are no longer used.
> > +
> >  What:		/sys/block/zram<id>/max_comp_streams
> >  Date:		February 2014
> >  Contact:	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
> > diff --git a/Documentation/blockdev/zram.txt b/Documentation/blockdev/zram.txt
> > index 0595c3f..e50e18b 100644
> > --- a/Documentation/blockdev/zram.txt
> > +++ b/Documentation/blockdev/zram.txt
> > @@ -89,6 +89,7 @@ size of the disk when not in use so a huge zram is wasteful.
> >  		num_writes
> >  		failed_reads
> >  		failed_writes
> > +		num_discards
> >  		invalid_io
> >  		notify_free
> >  		zero_pages
> > diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> > index d00831c..904e7a5 100644
> > --- a/drivers/block/zram/zram_drv.c
> > +++ b/drivers/block/zram/zram_drv.c
> > @@ -606,6 +606,7 @@ static void zram_bio_discard(struct zram *zram, u32 index,
> >  		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.num_discards);
> >  		index++;
> >  		n -= PAGE_SIZE;
> >  	}
> > @@ -866,6 +867,7 @@ ZRAM_ATTR_RO(num_reads);
> >  ZRAM_ATTR_RO(num_writes);
> >  ZRAM_ATTR_RO(failed_reads);
> >  ZRAM_ATTR_RO(failed_writes);
> > +ZRAM_ATTR_RO(num_discards);
> >  ZRAM_ATTR_RO(invalid_io);
> >  ZRAM_ATTR_RO(notify_free);
> >  ZRAM_ATTR_RO(zero_pages);
> > @@ -879,6 +881,7 @@ static struct attribute *zram_disk_attrs[] = {
> >  	&dev_attr_num_writes.attr,
> >  	&dev_attr_failed_reads.attr,
> >  	&dev_attr_failed_writes.attr,
> > +	&dev_attr_num_discards.attr,
> >  	&dev_attr_invalid_io.attr,
> >  	&dev_attr_notify_free.attr,
> >  	&dev_attr_zero_pages.attr,
> > diff --git a/drivers/block/zram/zram_drv.h b/drivers/block/zram/zram_drv.h
> > index e0f725c..2994aaf 100644
> > --- a/drivers/block/zram/zram_drv.h
> > +++ b/drivers/block/zram/zram_drv.h
> > @@ -86,6 +86,7 @@ struct zram_stats {
> >  	atomic64_t num_writes;	/* --do-- */
> >  	atomic64_t failed_reads;	/* can happen when memory is too low */
> >  	atomic64_t failed_writes;	/* can happen when memory is too low */
> > +	atomic64_t num_discards;	/* no. of discarded pages */
> >  	atomic64_t invalid_io;	/* non-page-aligned I/O requests */
> >  	atomic64_t notify_free;	/* no. of swap slot free notifications */
> >  	atomic64_t zero_pages;		/* no. of zero filled pages */
> > -- 
> > 2.0.1.474.g72c7794
> > 
> > 
> > --
> > To unsubscribe, send a message with 'unsubscribe linux-mm' in
> > the body to majordomo@kvack.org.  For more info on Linux MM,
> > see: http://www.linux-mm.org/ .
> > Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
> 
> -- 
> Kind regards,
> Minchan Kim
> 

RE: [PATCH v3] zram: add num_discards for discarded pages stat

[Chao Yu]

Hi Minchan,

> -----Original Message-----
> From: Minchan Kim [mailto:minchan@kernel.org]
> Sent: Monday, August 25, 2014 8:36 AM
> To: Chao Yu
> Cc: linux-kernel@vger.kernel.org; linux-mm@kvack.org; ngupta@vflare.org; 'Jerome Marchand';
> 'Sergey Senozhatsky'; 'Andrew Morton'
> Subject: Re: [PATCH v3] zram: add num_discards for discarded pages stat
> 
> Hello Chao,
> 
> On Fri, Aug 22, 2014 at 04:21:01PM +0800, Chao Yu wrote:
> > 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.
> 
> My concern is that how much we are able to know the effect of discard
> exactly with your patch.
> 
> The issue I can think of is zram-swap discard.
> Now, zram handles notification from VM to free duplicated copy between
> VM-owned memory and zRAM-owned's one so discarding for zram-swap might
> be pointless overhead but your stat indicates lots of free page discarded
> without real freeing so that user might think "We should keep enable
> swap discard for zRAM because the stat indicates it's really good".

Agreed.

> 
> In summary, wouldn't it better to have two?

Yeah, I'd like to.

> 
> num_discards,
> num_failed_discards?

It's good, but, IMHO, as it's not failed to discard pages due to inside
error of zRAM, How about show this information more positive by using:
num_discard_req,
num_discarded

Then user might think "We can keep on using real-time mode or batch mode
discard, because our freed pages are increased continuously shew by the
num_discarded with sending discard reqs each time.

How do you think?

Thanks,
Yu

> 
> For it, we should modify zram_free_page has return value.
> What do other guys think?
> 
> >
> > In this patch, we add num_discards to stat discarded pages, and export it to
> > sysfs for users.
> >
> > * From v1
> >  * Update zram document to show num_discards in statistics list.
> >
> > * From v2
> >  * Update description of this patch with clear goal.
> >
> > Signed-off-by: Chao Yu <chao2.yu@samsung.com>
> > ---
> >  Documentation/ABI/testing/sysfs-block-zram | 10 ++++++++++
> >  Documentation/blockdev/zram.txt            |  1 +
> >  drivers/block/zram/zram_drv.c              |  3 +++
> >  drivers/block/zram/zram_drv.h              |  1 +
> >  4 files changed, 15 insertions(+)
> >
> > diff --git a/Documentation/ABI/testing/sysfs-block-zram
> b/Documentation/ABI/testing/sysfs-block-zram
> > index 70ec992..fa8936e 100644
> > --- a/Documentation/ABI/testing/sysfs-block-zram
> > +++ b/Documentation/ABI/testing/sysfs-block-zram
> > @@ -57,6 +57,16 @@ Description:
> >  		The failed_writes file is read-only and specifies the number of
> >  		failed writes happened on this device.
> >
> > +
> > +What:		/sys/block/zram<id>/num_discards
> > +Date:		August 2014
> > +Contact:	Chao Yu <chao2.yu@samsung.com>
> > +Description:
> > +		The num_discards file is read-only and specifies the number of
> > +		physical blocks which are discarded by this device. These blocks
> > +		are included in discard request which is sended by filesystem as
> > +		the blocks are no longer used.
> > +
> >  What:		/sys/block/zram<id>/max_comp_streams
> >  Date:		February 2014
> >  Contact:	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
> > diff --git a/Documentation/blockdev/zram.txt b/Documentation/blockdev/zram.txt
> > index 0595c3f..e50e18b 100644
> > --- a/Documentation/blockdev/zram.txt
> > +++ b/Documentation/blockdev/zram.txt
> > @@ -89,6 +89,7 @@ size of the disk when not in use so a huge zram is wasteful.
> >  		num_writes
> >  		failed_reads
> >  		failed_writes
> > +		num_discards
> >  		invalid_io
> >  		notify_free
> >  		zero_pages
> > diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> > index d00831c..904e7a5 100644
> > --- a/drivers/block/zram/zram_drv.c
> > +++ b/drivers/block/zram/zram_drv.c
> > @@ -606,6 +606,7 @@ static void zram_bio_discard(struct zram *zram, u32 index,
> >  		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.num_discards);
> >  		index++;
> >  		n -= PAGE_SIZE;
> >  	}
> > @@ -866,6 +867,7 @@ ZRAM_ATTR_RO(num_reads);
> >  ZRAM_ATTR_RO(num_writes);
> >  ZRAM_ATTR_RO(failed_reads);
> >  ZRAM_ATTR_RO(failed_writes);
> > +ZRAM_ATTR_RO(num_discards);
> >  ZRAM_ATTR_RO(invalid_io);
> >  ZRAM_ATTR_RO(notify_free);
> >  ZRAM_ATTR_RO(zero_pages);
> > @@ -879,6 +881,7 @@ static struct attribute *zram_disk_attrs[] = {
> >  	&dev_attr_num_writes.attr,
> >  	&dev_attr_failed_reads.attr,
> >  	&dev_attr_failed_writes.attr,
> > +	&dev_attr_num_discards.attr,
> >  	&dev_attr_invalid_io.attr,
> >  	&dev_attr_notify_free.attr,
> >  	&dev_attr_zero_pages.attr,
> > diff --git a/drivers/block/zram/zram_drv.h b/drivers/block/zram/zram_drv.h
> > index e0f725c..2994aaf 100644
> > --- a/drivers/block/zram/zram_drv.h
> > +++ b/drivers/block/zram/zram_drv.h
> > @@ -86,6 +86,7 @@ struct zram_stats {
> >  	atomic64_t num_writes;	/* --do-- */
> >  	atomic64_t failed_reads;	/* can happen when memory is too low */
> >  	atomic64_t failed_writes;	/* can happen when memory is too low */
> > +	atomic64_t num_discards;	/* no. of discarded pages */
> >  	atomic64_t invalid_io;	/* non-page-aligned I/O requests */
> >  	atomic64_t notify_free;	/* no. of swap slot free notifications */
> >  	atomic64_t zero_pages;		/* no. of zero filled pages */
> > --
> > 2.0.1.474.g72c7794
> >
> >
> > --
> > To unsubscribe, send a message with 'unsubscribe linux-mm' in
> > the body to majordomo@kvack.org.  For more info on Linux MM,
> > see: http://www.linux-mm.org/ .
> > Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
> 
> --
> Kind regards,
> Minchan Kim

Re: [PATCH v3] zram: add num_discards for discarded pages stat

[Minchan Kim]

Hi,

On Mon, Aug 25, 2014 at 08:01:18PM +0900, Sergey Senozhatsky wrote:
> Hello,
> 
> On (08/25/14 09:36), Minchan Kim wrote:
> > Hello Chao,
> > 
> > On Fri, Aug 22, 2014 at 04:21:01PM +0800, Chao Yu wrote:
> > > 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.
> > 
> > My concern is that how much we are able to know the effect of discard
> > exactly with your patch.
> > 
> > The issue I can think of is zram-swap discard.
> > Now, zram handles notification from VM to free duplicated copy between
> > VM-owned memory and zRAM-owned's one so discarding for zram-swap might
> > be pointless overhead but your stat indicates lots of free page discarded
> > without real freeing 
> 
> this is why I've moved stats accounting to the place where actual
> zs_free() happens. and, frankly, I still would like to see the number
> of zs_free() calls, rather than the number of slot free notifications
> and REQ_DISCARD (or separately), because they all end up calling
> zs_free(). iow, despite the call path, from the user point of view
> they are just zs_free() -- the number of pages that's been freed by
> the 3rd party and we had have to deal with that.

My qeustion is that what user can do with the only real freeing count?
Could you give me a concret example?

It's a just number of real freeing count so if you were admin, what
do you expect from that? That's what I'd like to see in changelog.

> 
> > so that user might think "We should keep enable
> > swap discard for zRAM because the stat indicates it's really good".
> > 
> > In summary, wouldn't it better to have two?
> > 
> > num_discards,
> > num_failed_discards?
> 
> do we actully need this? the only value I can think of (perhaps I'm
> missing something) is that we can make sure that we need to support
> both slot free and REQ_DISCARDS, or we can leave only REQ_DISCARDS.
> is there anything else?

The secnario I imagined with two stat is how REQ_DISCARDS is effective
from swap layer. Normally, slot free logic is called in advance
when the page is zapped or swap read happens to avoid duplicate copy,
so discard request from swap space would be just overhead without
any benefit so we might guide zram-swap user don't use "swap -d".
Otherwise, as failed_discard ratio is low, it means it would be
better to remove swap slot free logic because swap discard works well
without slot free hint.(Although I don't think)

My point is I'm not saying you're wrong but adding a new stat is easy
and I need a compelling reason that how it can help users.

Thanks.

> 
> 	-ss
> 
> > For it, we should modify zram_free_page has return value.
> > What do other guys think?
> > 
> > > 
> > > In this patch, we add num_discards to stat discarded pages, and export it to
> > > sysfs for users.
> > > 
> > > * From v1
> > >  * Update zram document to show num_discards in statistics list.
> > > 
> > > * From v2
> > >  * Update description of this patch with clear goal.
> > > 
> > > Signed-off-by: Chao Yu <chao2.yu@samsung.com>
> > > ---
> > >  Documentation/ABI/testing/sysfs-block-zram | 10 ++++++++++
> > >  Documentation/blockdev/zram.txt            |  1 +
> > >  drivers/block/zram/zram_drv.c              |  3 +++
> > >  drivers/block/zram/zram_drv.h              |  1 +
> > >  4 files changed, 15 insertions(+)
> > > 
> > > diff --git a/Documentation/ABI/testing/sysfs-block-zram b/Documentation/ABI/testing/sysfs-block-zram
> > > index 70ec992..fa8936e 100644
> > > --- a/Documentation/ABI/testing/sysfs-block-zram
> > > +++ b/Documentation/ABI/testing/sysfs-block-zram
> > > @@ -57,6 +57,16 @@ Description:
> > >  		The failed_writes file is read-only and specifies the number of
> > >  		failed writes happened on this device.
> > >  
> > > +
> > > +What:		/sys/block/zram<id>/num_discards
> > > +Date:		August 2014
> > > +Contact:	Chao Yu <chao2.yu@samsung.com>
> > > +Description:
> > > +		The num_discards file is read-only and specifies the number of
> > > +		physical blocks which are discarded by this device. These blocks
> > > +		are included in discard request which is sended by filesystem as
> > > +		the blocks are no longer used.
> > > +
> > >  What:		/sys/block/zram<id>/max_comp_streams
> > >  Date:		February 2014
> > >  Contact:	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
> > > diff --git a/Documentation/blockdev/zram.txt b/Documentation/blockdev/zram.txt
> > > index 0595c3f..e50e18b 100644
> > > --- a/Documentation/blockdev/zram.txt
> > > +++ b/Documentation/blockdev/zram.txt
> > > @@ -89,6 +89,7 @@ size of the disk when not in use so a huge zram is wasteful.
> > >  		num_writes
> > >  		failed_reads
> > >  		failed_writes
> > > +		num_discards
> > >  		invalid_io
> > >  		notify_free
> > >  		zero_pages
> > > diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> > > index d00831c..904e7a5 100644
> > > --- a/drivers/block/zram/zram_drv.c
> > > +++ b/drivers/block/zram/zram_drv.c
> > > @@ -606,6 +606,7 @@ static void zram_bio_discard(struct zram *zram, u32 index,
> > >  		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.num_discards);
> > >  		index++;
> > >  		n -= PAGE_SIZE;
> > >  	}
> > > @@ -866,6 +867,7 @@ ZRAM_ATTR_RO(num_reads);
> > >  ZRAM_ATTR_RO(num_writes);
> > >  ZRAM_ATTR_RO(failed_reads);
> > >  ZRAM_ATTR_RO(failed_writes);
> > > +ZRAM_ATTR_RO(num_discards);
> > >  ZRAM_ATTR_RO(invalid_io);
> > >  ZRAM_ATTR_RO(notify_free);
> > >  ZRAM_ATTR_RO(zero_pages);
> > > @@ -879,6 +881,7 @@ static struct attribute *zram_disk_attrs[] = {
> > >  	&dev_attr_num_writes.attr,
> > >  	&dev_attr_failed_reads.attr,
> > >  	&dev_attr_failed_writes.attr,
> > > +	&dev_attr_num_discards.attr,
> > >  	&dev_attr_invalid_io.attr,
> > >  	&dev_attr_notify_free.attr,
> > >  	&dev_attr_zero_pages.attr,
> > > diff --git a/drivers/block/zram/zram_drv.h b/drivers/block/zram/zram_drv.h
> > > index e0f725c..2994aaf 100644
> > > --- a/drivers/block/zram/zram_drv.h
> > > +++ b/drivers/block/zram/zram_drv.h
> > > @@ -86,6 +86,7 @@ struct zram_stats {
> > >  	atomic64_t num_writes;	/* --do-- */
> > >  	atomic64_t failed_reads;	/* can happen when memory is too low */
> > >  	atomic64_t failed_writes;	/* can happen when memory is too low */
> > > +	atomic64_t num_discards;	/* no. of discarded pages */
> > >  	atomic64_t invalid_io;	/* non-page-aligned I/O requests */
> > >  	atomic64_t notify_free;	/* no. of swap slot free notifications */
> > >  	atomic64_t zero_pages;		/* no. of zero filled pages */
> > > -- 
> > > 2.0.1.474.g72c7794
> > > 
> > > 
> > > --
> > > To unsubscribe, send a message with 'unsubscribe linux-mm' in
> > > the body to majordomo@kvack.org.  For more info on Linux MM,
> > > see: http://www.linux-mm.org/ .
> > > Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
> > 
> > -- 
> > Kind regards,
> > Minchan Kim
> > 
> 
> --
> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> the body to majordomo@kvack.org.  For more info on Linux MM,
> see: http://www.linux-mm.org/ .
> Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

-- 
Kind regards,
Minchan Kim

Re: [PATCH v3] zram: add num_discards for discarded pages stat

[Minchan Kim]

On Tue, Aug 26, 2014 at 11:05:47AM +0800, Chao Yu wrote:
> Hi Minchan,
> 
> > -----Original Message-----
> > From: Minchan Kim [mailto:minchan@kernel.org]
> > Sent: Monday, August 25, 2014 8:36 AM
> > To: Chao Yu
> > Cc: linux-kernel@vger.kernel.org; linux-mm@kvack.org; ngupta@vflare.org; 'Jerome Marchand';
> > 'Sergey Senozhatsky'; 'Andrew Morton'
> > Subject: Re: [PATCH v3] zram: add num_discards for discarded pages stat
> > 
> > Hello Chao,
> > 
> > On Fri, Aug 22, 2014 at 04:21:01PM +0800, Chao Yu wrote:
> > > 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.
> > 
> > My concern is that how much we are able to know the effect of discard
> > exactly with your patch.
> > 
> > The issue I can think of is zram-swap discard.
> > Now, zram handles notification from VM to free duplicated copy between
> > VM-owned memory and zRAM-owned's one so discarding for zram-swap might
> > be pointless overhead but your stat indicates lots of free page discarded
> > without real freeing so that user might think "We should keep enable
> > swap discard for zRAM because the stat indicates it's really good".
> 
> Agreed.
> 
> > 
> > In summary, wouldn't it better to have two?
> 
> Yeah, I'd like to.
> 
> > 
> > num_discards,
> > num_failed_discards?
> 
> It's good, but, IMHO, as it's not failed to discard pages due to inside
> error of zRAM, How about show this information more positive by using:
> num_discard_req,
> num_discarded

Better.

> 
> Then user might think "We can keep on using real-time mode or batch mode
> discard, because our freed pages are increased continuously shew by the
> num_discarded with sending discard reqs each time.
> 
> How do you think?

It's good for your goal and my goal but I will wait Sergey's opinion.
Sergey, does it make sense to you, too?

> 
> Thanks,
> Yu
> 
> > 
> > For it, we should modify zram_free_page has return value.
> > What do other guys think?
> > 
> > >
> > > In this patch, we add num_discards to stat discarded pages, and export it to
> > > sysfs for users.
> > >
> > > * From v1
> > >  * Update zram document to show num_discards in statistics list.
> > >
> > > * From v2
> > >  * Update description of this patch with clear goal.
> > >
> > > Signed-off-by: Chao Yu <chao2.yu@samsung.com>
> > > ---
> > >  Documentation/ABI/testing/sysfs-block-zram | 10 ++++++++++
> > >  Documentation/blockdev/zram.txt            |  1 +
> > >  drivers/block/zram/zram_drv.c              |  3 +++
> > >  drivers/block/zram/zram_drv.h              |  1 +
> > >  4 files changed, 15 insertions(+)
> > >
> > > diff --git a/Documentation/ABI/testing/sysfs-block-zram
> > b/Documentation/ABI/testing/sysfs-block-zram
> > > index 70ec992..fa8936e 100644
> > > --- a/Documentation/ABI/testing/sysfs-block-zram
> > > +++ b/Documentation/ABI/testing/sysfs-block-zram
> > > @@ -57,6 +57,16 @@ Description:
> > >  		The failed_writes file is read-only and specifies the number of
> > >  		failed writes happened on this device.
> > >
> > > +
> > > +What:		/sys/block/zram<id>/num_discards
> > > +Date:		August 2014
> > > +Contact:	Chao Yu <chao2.yu@samsung.com>
> > > +Description:
> > > +		The num_discards file is read-only and specifies the number of
> > > +		physical blocks which are discarded by this device. These blocks
> > > +		are included in discard request which is sended by filesystem as
> > > +		the blocks are no longer used.
> > > +
> > >  What:		/sys/block/zram<id>/max_comp_streams
> > >  Date:		February 2014
> > >  Contact:	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
> > > diff --git a/Documentation/blockdev/zram.txt b/Documentation/blockdev/zram.txt
> > > index 0595c3f..e50e18b 100644
> > > --- a/Documentation/blockdev/zram.txt
> > > +++ b/Documentation/blockdev/zram.txt
> > > @@ -89,6 +89,7 @@ size of the disk when not in use so a huge zram is wasteful.
> > >  		num_writes
> > >  		failed_reads
> > >  		failed_writes
> > > +		num_discards
> > >  		invalid_io
> > >  		notify_free
> > >  		zero_pages
> > > diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> > > index d00831c..904e7a5 100644
> > > --- a/drivers/block/zram/zram_drv.c
> > > +++ b/drivers/block/zram/zram_drv.c
> > > @@ -606,6 +606,7 @@ static void zram_bio_discard(struct zram *zram, u32 index,
> > >  		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.num_discards);
> > >  		index++;
> > >  		n -= PAGE_SIZE;
> > >  	}
> > > @@ -866,6 +867,7 @@ ZRAM_ATTR_RO(num_reads);
> > >  ZRAM_ATTR_RO(num_writes);
> > >  ZRAM_ATTR_RO(failed_reads);
> > >  ZRAM_ATTR_RO(failed_writes);
> > > +ZRAM_ATTR_RO(num_discards);
> > >  ZRAM_ATTR_RO(invalid_io);
> > >  ZRAM_ATTR_RO(notify_free);
> > >  ZRAM_ATTR_RO(zero_pages);
> > > @@ -879,6 +881,7 @@ static struct attribute *zram_disk_attrs[] = {
> > >  	&dev_attr_num_writes.attr,
> > >  	&dev_attr_failed_reads.attr,
> > >  	&dev_attr_failed_writes.attr,
> > > +	&dev_attr_num_discards.attr,
> > >  	&dev_attr_invalid_io.attr,
> > >  	&dev_attr_notify_free.attr,
> > >  	&dev_attr_zero_pages.attr,
> > > diff --git a/drivers/block/zram/zram_drv.h b/drivers/block/zram/zram_drv.h
> > > index e0f725c..2994aaf 100644
> > > --- a/drivers/block/zram/zram_drv.h
> > > +++ b/drivers/block/zram/zram_drv.h
> > > @@ -86,6 +86,7 @@ struct zram_stats {
> > >  	atomic64_t num_writes;	/* --do-- */
> > >  	atomic64_t failed_reads;	/* can happen when memory is too low */
> > >  	atomic64_t failed_writes;	/* can happen when memory is too low */
> > > +	atomic64_t num_discards;	/* no. of discarded pages */
> > >  	atomic64_t invalid_io;	/* non-page-aligned I/O requests */
> > >  	atomic64_t notify_free;	/* no. of swap slot free notifications */
> > >  	atomic64_t zero_pages;		/* no. of zero filled pages */
> > > --
> > > 2.0.1.474.g72c7794
> > >
> > >
> > > --
> > > To unsubscribe, send a message with 'unsubscribe linux-mm' in
> > > the body to majordomo@kvack.org.  For more info on Linux MM,
> > > see: http://www.linux-mm.org/ .
> > > Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
> > 
> > --
> > Kind regards,
> > Minchan Kim
> 
> --
> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> the body to majordomo@kvack.org.  For more info on Linux MM,
> see: http://www.linux-mm.org/ .
> Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

-- 
Kind regards,
Minchan Kim

Re: [PATCH v3] zram: add num_discards for discarded pages stat

[Sergey Senozhatsky]

Hello,

On (08/26/14 14:08), Minchan Kim wrote:
> Hi,
> 
> On Mon, Aug 25, 2014 at 08:01:18PM +0900, Sergey Senozhatsky wrote:
> > Hello,
> > 
> > On (08/25/14 09:36), Minchan Kim wrote:
> > > Hello Chao,
> > > 
> > > On Fri, Aug 22, 2014 at 04:21:01PM +0800, Chao Yu wrote:
> > > > 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.
> > > 
> > > My concern is that how much we are able to know the effect of discard
> > > exactly with your patch.
> > > 
> > > The issue I can think of is zram-swap discard.
> > > Now, zram handles notification from VM to free duplicated copy between
> > > VM-owned memory and zRAM-owned's one so discarding for zram-swap might
> > > be pointless overhead but your stat indicates lots of free page discarded
> > > without real freeing 
> > 
> > this is why I've moved stats accounting to the place where actual
> > zs_free() happens. and, frankly, I still would like to see the number
> > of zs_free() calls, rather than the number of slot free notifications
> > and REQ_DISCARD (or separately), because they all end up calling
> > zs_free(). iow, despite the call path, from the user point of view
> > they are just zs_free() -- the number of pages that's been freed by
> > the 3rd party and we had have to deal with that.
> 
> My qeustion is that what user can do with the only real freeing count?
> Could you give me a concret example?

for !swap device case it's identicall to `num_discarded'.
for swap device case, it's a bit more complicated (less convenient) if
we actually can receive both slot free and delayed REQ_DISCARDs.

> It's a just number of real freeing count so if you were admin, what
> do you expect from that? That's what I'd like to see in changelog.
> 
> > 
> > > so that user might think "We should keep enable
> > > swap discard for zRAM because the stat indicates it's really good".
> > > 
> > > In summary, wouldn't it better to have two?
> > > 
> > > num_discards,
> > > num_failed_discards?
> > 
> > do we actully need this? the only value I can think of (perhaps I'm
> > missing something) is that we can make sure that we need to support
> > both slot free and REQ_DISCARDS, or we can leave only REQ_DISCARDS.
> > is there anything else?
> 
> The secnario I imagined with two stat is how REQ_DISCARDS is effective
> from swap layer. Normally, slot free logic is called in advance
> when the page is zapped or swap read happens to avoid duplicate copy,
> so discard request from swap space would be just overhead without
> any benefit so we might guide zram-swap user don't use "swap -d".
> Otherwise, as failed_discard ratio is low, it means it would be
> better to remove swap slot free logic because swap discard works well
> without slot free hint.(Although I don't think)

yes, so it looks like it is a developer's stat - to make some
observations and to come up with some decisions. do we really
want to put it into release?


I'm not strongly against and we can proceed with Chao's patch.

	-ss

> My point is I'm not saying you're wrong but adding a new stat is easy
> and I need a compelling reason that how it can help users.
> 
> Thanks.
> 
> > 
> > 	-ss
> > 
> > > For it, we should modify zram_free_page has return value.
> > > What do other guys think?
> > > 
> > > > 
> > > > In this patch, we add num_discards to stat discarded pages, and export it to
> > > > sysfs for users.
> > > > 
> > > > * From v1
> > > >  * Update zram document to show num_discards in statistics list.
> > > > 
> > > > * From v2
> > > >  * Update description of this patch with clear goal.
> > > > 
> > > > Signed-off-by: Chao Yu <chao2.yu@samsung.com>
> > > > ---
> > > >  Documentation/ABI/testing/sysfs-block-zram | 10 ++++++++++
> > > >  Documentation/blockdev/zram.txt            |  1 +
> > > >  drivers/block/zram/zram_drv.c              |  3 +++
> > > >  drivers/block/zram/zram_drv.h              |  1 +
> > > >  4 files changed, 15 insertions(+)
> > > > 
> > > > diff --git a/Documentation/ABI/testing/sysfs-block-zram b/Documentation/ABI/testing/sysfs-block-zram
> > > > index 70ec992..fa8936e 100644
> > > > --- a/Documentation/ABI/testing/sysfs-block-zram
> > > > +++ b/Documentation/ABI/testing/sysfs-block-zram
> > > > @@ -57,6 +57,16 @@ Description:
> > > >  		The failed_writes file is read-only and specifies the number of
> > > >  		failed writes happened on this device.
> > > >  
> > > > +
> > > > +What:		/sys/block/zram<id>/num_discards
> > > > +Date:		August 2014
> > > > +Contact:	Chao Yu <chao2.yu@samsung.com>
> > > > +Description:
> > > > +		The num_discards file is read-only and specifies the number of
> > > > +		physical blocks which are discarded by this device. These blocks
> > > > +		are included in discard request which is sended by filesystem as
> > > > +		the blocks are no longer used.
> > > > +
> > > >  What:		/sys/block/zram<id>/max_comp_streams
> > > >  Date:		February 2014
> > > >  Contact:	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
> > > > diff --git a/Documentation/blockdev/zram.txt b/Documentation/blockdev/zram.txt
> > > > index 0595c3f..e50e18b 100644
> > > > --- a/Documentation/blockdev/zram.txt
> > > > +++ b/Documentation/blockdev/zram.txt
> > > > @@ -89,6 +89,7 @@ size of the disk when not in use so a huge zram is wasteful.
> > > >  		num_writes
> > > >  		failed_reads
> > > >  		failed_writes
> > > > +		num_discards
> > > >  		invalid_io
> > > >  		notify_free
> > > >  		zero_pages
> > > > diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> > > > index d00831c..904e7a5 100644
> > > > --- a/drivers/block/zram/zram_drv.c
> > > > +++ b/drivers/block/zram/zram_drv.c
> > > > @@ -606,6 +606,7 @@ static void zram_bio_discard(struct zram *zram, u32 index,
> > > >  		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.num_discards);
> > > >  		index++;
> > > >  		n -= PAGE_SIZE;
> > > >  	}
> > > > @@ -866,6 +867,7 @@ ZRAM_ATTR_RO(num_reads);
> > > >  ZRAM_ATTR_RO(num_writes);
> > > >  ZRAM_ATTR_RO(failed_reads);
> > > >  ZRAM_ATTR_RO(failed_writes);
> > > > +ZRAM_ATTR_RO(num_discards);
> > > >  ZRAM_ATTR_RO(invalid_io);
> > > >  ZRAM_ATTR_RO(notify_free);
> > > >  ZRAM_ATTR_RO(zero_pages);
> > > > @@ -879,6 +881,7 @@ static struct attribute *zram_disk_attrs[] = {
> > > >  	&dev_attr_num_writes.attr,
> > > >  	&dev_attr_failed_reads.attr,
> > > >  	&dev_attr_failed_writes.attr,
> > > > +	&dev_attr_num_discards.attr,
> > > >  	&dev_attr_invalid_io.attr,
> > > >  	&dev_attr_notify_free.attr,
> > > >  	&dev_attr_zero_pages.attr,
> > > > diff --git a/drivers/block/zram/zram_drv.h b/drivers/block/zram/zram_drv.h
> > > > index e0f725c..2994aaf 100644
> > > > --- a/drivers/block/zram/zram_drv.h
> > > > +++ b/drivers/block/zram/zram_drv.h
> > > > @@ -86,6 +86,7 @@ struct zram_stats {
> > > >  	atomic64_t num_writes;	/* --do-- */
> > > >  	atomic64_t failed_reads;	/* can happen when memory is too low */
> > > >  	atomic64_t failed_writes;	/* can happen when memory is too low */
> > > > +	atomic64_t num_discards;	/* no. of discarded pages */
> > > >  	atomic64_t invalid_io;	/* non-page-aligned I/O requests */
> > > >  	atomic64_t notify_free;	/* no. of swap slot free notifications */
> > > >  	atomic64_t zero_pages;		/* no. of zero filled pages */
> > > > -- 
> > > > 2.0.1.474.g72c7794
> > > > 
> > > > 
> > > > --
> > > > To unsubscribe, send a message with 'unsubscribe linux-mm' in
> > > > the body to majordomo@kvack.org.  For more info on Linux MM,
> > > > see: http://www.linux-mm.org/ .
> > > > Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
> > > 
> > > -- 
> > > Kind regards,
> > > Minchan Kim
> > > 
> > 
> > --
> > To unsubscribe, send a message with 'unsubscribe linux-mm' in
> > the body to majordomo@kvack.org.  For more info on Linux MM,
> > see: http://www.linux-mm.org/ .
> > Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
> 
> -- 
> Kind regards,
> Minchan Kim
> 

Re: [PATCH v3] zram: add num_discards for discarded pages stat

[Minchan Kim]

Hello Sergey,

First of all, Sorry for late response.

On Tue, Aug 26, 2014 at 11:15:43PM +0900, Sergey Senozhatsky wrote:
> Hello,
> 
> On (08/26/14 14:08), Minchan Kim wrote:
> > Hi,
> > 
> > On Mon, Aug 25, 2014 at 08:01:18PM +0900, Sergey Senozhatsky wrote:
> > > Hello,
> > > 
> > > On (08/25/14 09:36), Minchan Kim wrote:
> > > > Hello Chao,
> > > > 
> > > > On Fri, Aug 22, 2014 at 04:21:01PM +0800, Chao Yu wrote:
> > > > > 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.
> > > > 
> > > > My concern is that how much we are able to know the effect of discard
> > > > exactly with your patch.
> > > > 
> > > > The issue I can think of is zram-swap discard.
> > > > Now, zram handles notification from VM to free duplicated copy between
> > > > VM-owned memory and zRAM-owned's one so discarding for zram-swap might
> > > > be pointless overhead but your stat indicates lots of free page discarded
> > > > without real freeing 
> > > 
> > > this is why I've moved stats accounting to the place where actual
> > > zs_free() happens. and, frankly, I still would like to see the number
> > > of zs_free() calls, rather than the number of slot free notifications
> > > and REQ_DISCARD (or separately), because they all end up calling
> > > zs_free(). iow, despite the call path, from the user point of view
> > > they are just zs_free() -- the number of pages that's been freed by
> > > the 3rd party and we had have to deal with that.
> > 
> > My qeustion is that what user can do with the only real freeing count?
> > Could you give me a concret example?
> 
> for !swap device case it's identicall to `num_discarded'.
> for swap device case, it's a bit more complicated (less convenient) if
> we actually can receive both slot free and delayed REQ_DISCARDs.
> 
> > It's a just number of real freeing count so if you were admin, what
> > do you expect from that? That's what I'd like to see in changelog.
> > 
> > > 
> > > > so that user might think "We should keep enable
> > > > swap discard for zRAM because the stat indicates it's really good".
> > > > 
> > > > In summary, wouldn't it better to have two?
> > > > 
> > > > num_discards,
> > > > num_failed_discards?
> > > 
> > > do we actully need this? the only value I can think of (perhaps I'm
> > > missing something) is that we can make sure that we need to support
> > > both slot free and REQ_DISCARDS, or we can leave only REQ_DISCARDS.
> > > is there anything else?
> > 
> > The secnario I imagined with two stat is how REQ_DISCARDS is effective
> > from swap layer. Normally, slot free logic is called in advance
> > when the page is zapped or swap read happens to avoid duplicate copy,
> > so discard request from swap space would be just overhead without
> > any benefit so we might guide zram-swap user don't use "swap -d".
> > Otherwise, as failed_discard ratio is low, it means it would be
> > better to remove swap slot free logic because swap discard works well
> > without slot free hint.(Although I don't think)
> 
> yes, so it looks like it is a developer's stat - to make some
> observations and to come up with some decisions. do we really
> want to put it into release?

Agree. I was too specific for my purpose and it couldn't be
a compelling reason to make it export for general purpose.

Actually, discard req sent by swap for getting free cluster
shouldn't be success(i,e num_discarded should be zero) because
zram_slot_free_notify will always free the duplicated copy
in advance so user don't have any gain with 'swapon -d'.

Now, I agree with you that we shouldn't add more stat without
compelling reason so it would be better to rename notify_free
with discarded and move it in zram_free_page like your patch.
https://lkml.org/lkml/2014/8/21/294

I will ask to Andrew to revert Chao's patch and pick your patch
after a few days unless Chao has another opinion.


> 
> 
> I'm not strongly against and we can proceed with Chao's patch.
> 
> 	-ss
> 
> > My point is I'm not saying you're wrong but adding a new stat is easy
> > and I need a compelling reason that how it can help users.
> > 
> > Thanks.
> > 
> > > 
> > > 	-ss
> > > 
> > > > For it, we should modify zram_free_page has return value.
> > > > What do other guys think?
> > > > 
> > > > > 
> > > > > In this patch, we add num_discards to stat discarded pages, and export it to
> > > > > sysfs for users.
> > > > > 
> > > > > * From v1
> > > > >  * Update zram document to show num_discards in statistics list.
> > > > > 
> > > > > * From v2
> > > > >  * Update description of this patch with clear goal.
> > > > > 
> > > > > Signed-off-by: Chao Yu <chao2.yu@samsung.com>
> > > > > ---
> > > > >  Documentation/ABI/testing/sysfs-block-zram | 10 ++++++++++
> > > > >  Documentation/blockdev/zram.txt            |  1 +
> > > > >  drivers/block/zram/zram_drv.c              |  3 +++
> > > > >  drivers/block/zram/zram_drv.h              |  1 +
> > > > >  4 files changed, 15 insertions(+)
> > > > > 
> > > > > diff --git a/Documentation/ABI/testing/sysfs-block-zram b/Documentation/ABI/testing/sysfs-block-zram
> > > > > index 70ec992..fa8936e 100644
> > > > > --- a/Documentation/ABI/testing/sysfs-block-zram
> > > > > +++ b/Documentation/ABI/testing/sysfs-block-zram
> > > > > @@ -57,6 +57,16 @@ Description:
> > > > >  		The failed_writes file is read-only and specifies the number of
> > > > >  		failed writes happened on this device.
> > > > >  
> > > > > +
> > > > > +What:		/sys/block/zram<id>/num_discards
> > > > > +Date:		August 2014
> > > > > +Contact:	Chao Yu <chao2.yu@samsung.com>
> > > > > +Description:
> > > > > +		The num_discards file is read-only and specifies the number of
> > > > > +		physical blocks which are discarded by this device. These blocks
> > > > > +		are included in discard request which is sended by filesystem as
> > > > > +		the blocks are no longer used.
> > > > > +
> > > > >  What:		/sys/block/zram<id>/max_comp_streams
> > > > >  Date:		February 2014
> > > > >  Contact:	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
> > > > > diff --git a/Documentation/blockdev/zram.txt b/Documentation/blockdev/zram.txt
> > > > > index 0595c3f..e50e18b 100644
> > > > > --- a/Documentation/blockdev/zram.txt
> > > > > +++ b/Documentation/blockdev/zram.txt
> > > > > @@ -89,6 +89,7 @@ size of the disk when not in use so a huge zram is wasteful.
> > > > >  		num_writes
> > > > >  		failed_reads
> > > > >  		failed_writes
> > > > > +		num_discards
> > > > >  		invalid_io
> > > > >  		notify_free
> > > > >  		zero_pages
> > > > > diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> > > > > index d00831c..904e7a5 100644
> > > > > --- a/drivers/block/zram/zram_drv.c
> > > > > +++ b/drivers/block/zram/zram_drv.c
> > > > > @@ -606,6 +606,7 @@ static void zram_bio_discard(struct zram *zram, u32 index,
> > > > >  		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.num_discards);
> > > > >  		index++;
> > > > >  		n -= PAGE_SIZE;
> > > > >  	}
> > > > > @@ -866,6 +867,7 @@ ZRAM_ATTR_RO(num_reads);
> > > > >  ZRAM_ATTR_RO(num_writes);
> > > > >  ZRAM_ATTR_RO(failed_reads);
> > > > >  ZRAM_ATTR_RO(failed_writes);
> > > > > +ZRAM_ATTR_RO(num_discards);
> > > > >  ZRAM_ATTR_RO(invalid_io);
> > > > >  ZRAM_ATTR_RO(notify_free);
> > > > >  ZRAM_ATTR_RO(zero_pages);
> > > > > @@ -879,6 +881,7 @@ static struct attribute *zram_disk_attrs[] = {
> > > > >  	&dev_attr_num_writes.attr,
> > > > >  	&dev_attr_failed_reads.attr,
> > > > >  	&dev_attr_failed_writes.attr,
> > > > > +	&dev_attr_num_discards.attr,
> > > > >  	&dev_attr_invalid_io.attr,
> > > > >  	&dev_attr_notify_free.attr,
> > > > >  	&dev_attr_zero_pages.attr,
> > > > > diff --git a/drivers/block/zram/zram_drv.h b/drivers/block/zram/zram_drv.h
> > > > > index e0f725c..2994aaf 100644
> > > > > --- a/drivers/block/zram/zram_drv.h
> > > > > +++ b/drivers/block/zram/zram_drv.h
> > > > > @@ -86,6 +86,7 @@ struct zram_stats {
> > > > >  	atomic64_t num_writes;	/* --do-- */
> > > > >  	atomic64_t failed_reads;	/* can happen when memory is too low */
> > > > >  	atomic64_t failed_writes;	/* can happen when memory is too low */
> > > > > +	atomic64_t num_discards;	/* no. of discarded pages */
> > > > >  	atomic64_t invalid_io;	/* non-page-aligned I/O requests */
> > > > >  	atomic64_t notify_free;	/* no. of swap slot free notifications */
> > > > >  	atomic64_t zero_pages;		/* no. of zero filled pages */
> > > > > -- 
> > > > > 2.0.1.474.g72c7794
> > > > > 
> > > > > 
> > > > > --
> > > > > To unsubscribe, send a message with 'unsubscribe linux-mm' in
> > > > > the body to majordomo@kvack.org.  For more info on Linux MM,
> > > > > see: http://www.linux-mm.org/ .
> > > > > Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
> > > > 
> > > > -- 
> > > > Kind regards,
> > > > Minchan Kim
> > > > 
> > > 
> > > --
> > > To unsubscribe, send a message with 'unsubscribe linux-mm' in
> > > the body to majordomo@kvack.org.  For more info on Linux MM,
> > > see: http://www.linux-mm.org/ .
> > > Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
> > 
> > -- 
> > Kind regards,
> > Minchan Kim
> > 
> 
> --
> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> the body to majordomo@kvack.org.  For more info on Linux MM,
> see: http://www.linux-mm.org/ .
> Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

-- 
Kind regards,
Minchan Kim

Re: [PATCH v3] zram: add num_discards for discarded pages stat

[Sergey Senozhatsky]

Hello,

On (09/04/14 11:25), Minchan Kim wrote:
> Hello Sergey,
> 
> First of all, Sorry for late response.
> 
> On Tue, Aug 26, 2014 at 11:15:43PM +0900, Sergey Senozhatsky wrote:
> > Hello,
> > 
> > On (08/26/14 14:08), Minchan Kim wrote:
> > > Hi,
> > > 
> > > On Mon, Aug 25, 2014 at 08:01:18PM +0900, Sergey Senozhatsky wrote:
> > > > Hello,
> > > > 
> > > > On (08/25/14 09:36), Minchan Kim wrote:
> > > > > Hello Chao,
> > > > > 
> > > > > On Fri, Aug 22, 2014 at 04:21:01PM +0800, Chao Yu wrote:
> > > > > > 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.
> > > > > 
> > > > > My concern is that how much we are able to know the effect of discard
> > > > > exactly with your patch.
> > > > > 
> > > > > The issue I can think of is zram-swap discard.
> > > > > Now, zram handles notification from VM to free duplicated copy between
> > > > > VM-owned memory and zRAM-owned's one so discarding for zram-swap might
> > > > > be pointless overhead but your stat indicates lots of free page discarded
> > > > > without real freeing 
> > > > 
> > > > this is why I've moved stats accounting to the place where actual
> > > > zs_free() happens. and, frankly, I still would like to see the number
> > > > of zs_free() calls, rather than the number of slot free notifications
> > > > and REQ_DISCARD (or separately), because they all end up calling
> > > > zs_free(). iow, despite the call path, from the user point of view
> > > > they are just zs_free() -- the number of pages that's been freed by
> > > > the 3rd party and we had have to deal with that.
> > > 
> > > My qeustion is that what user can do with the only real freeing count?
> > > Could you give me a concret example?
> > 
> > for !swap device case it's identicall to `num_discarded'.
> > for swap device case, it's a bit more complicated (less convenient) if
> > we actually can receive both slot free and delayed REQ_DISCARDs.
> > 
> > > It's a just number of real freeing count so if you were admin, what
> > > do you expect from that? That's what I'd like to see in changelog.
> > > 
> > > > 
> > > > > so that user might think "We should keep enable
> > > > > swap discard for zRAM because the stat indicates it's really good".
> > > > > 
> > > > > In summary, wouldn't it better to have two?
> > > > > 
> > > > > num_discards,
> > > > > num_failed_discards?
> > > > 
> > > > do we actully need this? the only value I can think of (perhaps I'm
> > > > missing something) is that we can make sure that we need to support
> > > > both slot free and REQ_DISCARDS, or we can leave only REQ_DISCARDS.
> > > > is there anything else?
> > > 
> > > The secnario I imagined with two stat is how REQ_DISCARDS is effective
> > > from swap layer. Normally, slot free logic is called in advance
> > > when the page is zapped or swap read happens to avoid duplicate copy,
> > > so discard request from swap space would be just overhead without
> > > any benefit so we might guide zram-swap user don't use "swap -d".
> > > Otherwise, as failed_discard ratio is low, it means it would be
> > > better to remove swap slot free logic because swap discard works well
> > > without slot free hint.(Although I don't think)
> > 
> > yes, so it looks like it is a developer's stat - to make some
> > observations and to come up with some decisions. do we really
> > want to put it into release?
> 
> Agree. I was too specific for my purpose and it couldn't be
> a compelling reason to make it export for general purpose.
> 
> Actually, discard req sent by swap for getting free cluster
> shouldn't be success(i,e num_discarded should be zero) because
> zram_slot_free_notify will always free the duplicated copy
> in advance so user don't have any gain with 'swapon -d'.
> 
> Now, I agree with you that we shouldn't add more stat without
> compelling reason so it would be better to rename notify_free
> with discarded and move it in zram_free_page like your patch.
> https://lkml.org/lkml/2014/8/21/294
> 
> I will ask to Andrew to revert Chao's patch and pick your patch
> after a few days unless Chao has another opinion.
> 

no problem.

I, probably, was not clear enough. one of my objections was that
it is really easy to add a new stat file, and surprisingly hard to
remove it later, even a temporary one. because it's almost impossible
to beat the "someone might use it" argument.
just a side note, my whole impresion is that some of the stats, that we
export, belongs to /proc/diskstats, /sys/block/zramX/stat and frieds.
I didn't check but I think that some handy tools like iostat/etc are
using these files. though I have no idea how often (if ever) people
want to track (or at least see) zram in iostat or similar tools.



please let me know if I have to resend the patch.

	-ss

> > 
> > 
> > I'm not strongly against and we can proceed with Chao's patch.
> > 
> > 	-ss
> > 
> > > My point is I'm not saying you're wrong but adding a new stat is easy
> > > and I need a compelling reason that how it can help users.
> > > 
> > > Thanks.
> > > 
> > > > 
> > > > 	-ss
> > > > 
> > > > > For it, we should modify zram_free_page has return value.
> > > > > What do other guys think?
> > > > > 
> > > > > > 
> > > > > > In this patch, we add num_discards to stat discarded pages, and export it to
> > > > > > sysfs for users.
> > > > > > 
> > > > > > * From v1
> > > > > >  * Update zram document to show num_discards in statistics list.
> > > > > > 
> > > > > > * From v2
> > > > > >  * Update description of this patch with clear goal.
> > > > > > 
> > > > > > Signed-off-by: Chao Yu <chao2.yu@samsung.com>
> > > > > > ---
> > > > > >  Documentation/ABI/testing/sysfs-block-zram | 10 ++++++++++
> > > > > >  Documentation/blockdev/zram.txt            |  1 +
> > > > > >  drivers/block/zram/zram_drv.c              |  3 +++
> > > > > >  drivers/block/zram/zram_drv.h              |  1 +
> > > > > >  4 files changed, 15 insertions(+)
> > > > > > 
> > > > > > diff --git a/Documentation/ABI/testing/sysfs-block-zram b/Documentation/ABI/testing/sysfs-block-zram
> > > > > > index 70ec992..fa8936e 100644
> > > > > > --- a/Documentation/ABI/testing/sysfs-block-zram
> > > > > > +++ b/Documentation/ABI/testing/sysfs-block-zram
> > > > > > @@ -57,6 +57,16 @@ Description:
> > > > > >  		The failed_writes file is read-only and specifies the number of
> > > > > >  		failed writes happened on this device.
> > > > > >  
> > > > > > +
> > > > > > +What:		/sys/block/zram<id>/num_discards
> > > > > > +Date:		August 2014
> > > > > > +Contact:	Chao Yu <chao2.yu@samsung.com>
> > > > > > +Description:
> > > > > > +		The num_discards file is read-only and specifies the number of
> > > > > > +		physical blocks which are discarded by this device. These blocks
> > > > > > +		are included in discard request which is sended by filesystem as
> > > > > > +		the blocks are no longer used.
> > > > > > +
> > > > > >  What:		/sys/block/zram<id>/max_comp_streams
> > > > > >  Date:		February 2014
> > > > > >  Contact:	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
> > > > > > diff --git a/Documentation/blockdev/zram.txt b/Documentation/blockdev/zram.txt
> > > > > > index 0595c3f..e50e18b 100644
> > > > > > --- a/Documentation/blockdev/zram.txt
> > > > > > +++ b/Documentation/blockdev/zram.txt
> > > > > > @@ -89,6 +89,7 @@ size of the disk when not in use so a huge zram is wasteful.
> > > > > >  		num_writes
> > > > > >  		failed_reads
> > > > > >  		failed_writes
> > > > > > +		num_discards
> > > > > >  		invalid_io
> > > > > >  		notify_free
> > > > > >  		zero_pages
> > > > > > diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> > > > > > index d00831c..904e7a5 100644
> > > > > > --- a/drivers/block/zram/zram_drv.c
> > > > > > +++ b/drivers/block/zram/zram_drv.c
> > > > > > @@ -606,6 +606,7 @@ static void zram_bio_discard(struct zram *zram, u32 index,
> > > > > >  		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.num_discards);
> > > > > >  		index++;
> > > > > >  		n -= PAGE_SIZE;
> > > > > >  	}
> > > > > > @@ -866,6 +867,7 @@ ZRAM_ATTR_RO(num_reads);
> > > > > >  ZRAM_ATTR_RO(num_writes);
> > > > > >  ZRAM_ATTR_RO(failed_reads);
> > > > > >  ZRAM_ATTR_RO(failed_writes);
> > > > > > +ZRAM_ATTR_RO(num_discards);
> > > > > >  ZRAM_ATTR_RO(invalid_io);
> > > > > >  ZRAM_ATTR_RO(notify_free);
> > > > > >  ZRAM_ATTR_RO(zero_pages);
> > > > > > @@ -879,6 +881,7 @@ static struct attribute *zram_disk_attrs[] = {
> > > > > >  	&dev_attr_num_writes.attr,
> > > > > >  	&dev_attr_failed_reads.attr,
> > > > > >  	&dev_attr_failed_writes.attr,
> > > > > > +	&dev_attr_num_discards.attr,
> > > > > >  	&dev_attr_invalid_io.attr,
> > > > > >  	&dev_attr_notify_free.attr,
> > > > > >  	&dev_attr_zero_pages.attr,
> > > > > > diff --git a/drivers/block/zram/zram_drv.h b/drivers/block/zram/zram_drv.h
> > > > > > index e0f725c..2994aaf 100644
> > > > > > --- a/drivers/block/zram/zram_drv.h
> > > > > > +++ b/drivers/block/zram/zram_drv.h
> > > > > > @@ -86,6 +86,7 @@ struct zram_stats {
> > > > > >  	atomic64_t num_writes;	/* --do-- */
> > > > > >  	atomic64_t failed_reads;	/* can happen when memory is too low */
> > > > > >  	atomic64_t failed_writes;	/* can happen when memory is too low */
> > > > > > +	atomic64_t num_discards;	/* no. of discarded pages */
> > > > > >  	atomic64_t invalid_io;	/* non-page-aligned I/O requests */
> > > > > >  	atomic64_t notify_free;	/* no. of swap slot free notifications */
> > > > > >  	atomic64_t zero_pages;		/* no. of zero filled pages */
> > > > > > -- 
> > > > > > 2.0.1.474.g72c7794
> > > > > > 
> > > > > > 
> > > > > > --
> > > > > > To unsubscribe, send a message with 'unsubscribe linux-mm' in
> > > > > > the body to majordomo@kvack.org.  For more info on Linux MM,
> > > > > > see: http://www.linux-mm.org/ .
> > > > > > Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
> > > > > 
> > > > > -- 
> > > > > Kind regards,
> > > > > Minchan Kim
> > > > > 
> > > > 
> > > > --
> > > > To unsubscribe, send a message with 'unsubscribe linux-mm' in
> > > > the body to majordomo@kvack.org.  For more info on Linux MM,
> > > > see: http://www.linux-mm.org/ .
> > > > Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
> > > 
> > > -- 
> > > Kind regards,
> > > Minchan Kim
> > > 
> > 
> > --
> > To unsubscribe, send a message with 'unsubscribe linux-mm' in
> > the body to majordomo@kvack.org.  For more info on Linux MM,
> > see: http://www.linux-mm.org/ .
> > Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
> 
> -- 
> Kind regards,
> Minchan Kim
> 

Re: [PATCH v3] zram: add num_discards for discarded pages stat

[Minchan Kim]

Hi Sergey,

On Fri, Sep 05, 2014 at 12:43:31AM +0900, Sergey Senozhatsky wrote:
> Hello,
> 
> On (09/04/14 11:25), Minchan Kim wrote:
> > Hello Sergey,
> > 
> > First of all, Sorry for late response.
> > 
> > On Tue, Aug 26, 2014 at 11:15:43PM +0900, Sergey Senozhatsky wrote:
> > > Hello,
> > > 
> > > On (08/26/14 14:08), Minchan Kim wrote:
> > > > Hi,
> > > > 
> > > > On Mon, Aug 25, 2014 at 08:01:18PM +0900, Sergey Senozhatsky wrote:
> > > > > Hello,
> > > > > 
> > > > > On (08/25/14 09:36), Minchan Kim wrote:
> > > > > > Hello Chao,
> > > > > > 
> > > > > > On Fri, Aug 22, 2014 at 04:21:01PM +0800, Chao Yu wrote:
> > > > > > > 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.
> > > > > > 
> > > > > > My concern is that how much we are able to know the effect of discard
> > > > > > exactly with your patch.
> > > > > > 
> > > > > > The issue I can think of is zram-swap discard.
> > > > > > Now, zram handles notification from VM to free duplicated copy between
> > > > > > VM-owned memory and zRAM-owned's one so discarding for zram-swap might
> > > > > > be pointless overhead but your stat indicates lots of free page discarded
> > > > > > without real freeing 
> > > > > 
> > > > > this is why I've moved stats accounting to the place where actual
> > > > > zs_free() happens. and, frankly, I still would like to see the number
> > > > > of zs_free() calls, rather than the number of slot free notifications
> > > > > and REQ_DISCARD (or separately), because they all end up calling
> > > > > zs_free(). iow, despite the call path, from the user point of view
> > > > > they are just zs_free() -- the number of pages that's been freed by
> > > > > the 3rd party and we had have to deal with that.
> > > > 
> > > > My qeustion is that what user can do with the only real freeing count?
> > > > Could you give me a concret example?
> > > 
> > > for !swap device case it's identicall to `num_discarded'.
> > > for swap device case, it's a bit more complicated (less convenient) if
> > > we actually can receive both slot free and delayed REQ_DISCARDs.
> > > 
> > > > It's a just number of real freeing count so if you were admin, what
> > > > do you expect from that? That's what I'd like to see in changelog.
> > > > 
> > > > > 
> > > > > > so that user might think "We should keep enable
> > > > > > swap discard for zRAM because the stat indicates it's really good".
> > > > > > 
> > > > > > In summary, wouldn't it better to have two?
> > > > > > 
> > > > > > num_discards,
> > > > > > num_failed_discards?
> > > > > 
> > > > > do we actully need this? the only value I can think of (perhaps I'm
> > > > > missing something) is that we can make sure that we need to support
> > > > > both slot free and REQ_DISCARDS, or we can leave only REQ_DISCARDS.
> > > > > is there anything else?
> > > > 
> > > > The secnario I imagined with two stat is how REQ_DISCARDS is effective
> > > > from swap layer. Normally, slot free logic is called in advance
> > > > when the page is zapped or swap read happens to avoid duplicate copy,
> > > > so discard request from swap space would be just overhead without
> > > > any benefit so we might guide zram-swap user don't use "swap -d".
> > > > Otherwise, as failed_discard ratio is low, it means it would be
> > > > better to remove swap slot free logic because swap discard works well
> > > > without slot free hint.(Although I don't think)
> > > 
> > > yes, so it looks like it is a developer's stat - to make some
> > > observations and to come up with some decisions. do we really
> > > want to put it into release?
> > 
> > Agree. I was too specific for my purpose and it couldn't be
> > a compelling reason to make it export for general purpose.
> > 
> > Actually, discard req sent by swap for getting free cluster
> > shouldn't be success(i,e num_discarded should be zero) because
> > zram_slot_free_notify will always free the duplicated copy
> > in advance so user don't have any gain with 'swapon -d'.
> > 
> > Now, I agree with you that we shouldn't add more stat without
> > compelling reason so it would be better to rename notify_free
> > with discarded and move it in zram_free_page like your patch.
> > https://lkml.org/lkml/2014/8/21/294
> > 
> > I will ask to Andrew to revert Chao's patch and pick your patch
> > after a few days unless Chao has another opinion.
> > 
> 
> no problem.
> 
> I, probably, was not clear enough. one of my objections was that
> it is really easy to add a new stat file, and surprisingly hard to
> remove it later, even a temporary one. because it's almost impossible
> to beat the "someone might use it" argument.

Almost true but it's not the case for sysfs.
Documentation/sysfs-rulies.txt says

"The kernel-exported sysfs exports internal kernel implementation details
and depends on internal kernel structures and layout. It is agreed upon
by the kernel developers that the Linux kernel does not provide a stable
internal API. Therefore, there are aspects of the sysfs interface that
may not be stable across kernel releases."

So, we could decide a schedule when we removes and warn to user
if they try to see that. If any serious issue doesn't appear until
the deadline, we would remove it then.

You could read Documentation/ABI/README.


> just a side note, my whole impresion is that some of the stats, that we
> export, belongs to /proc/diskstats, /sys/block/zramX/stat and frieds.
> I didn't check but I think that some handy tools like iostat/etc are
> using these files. though I have no idea how often (if ever) people
> want to track (or at least see) zram in iostat or similar tools.

Yeb, I knew it and wanted to clean it up in my spare time but
patches are welcome!

> 
> 
> 
> please let me know if I have to resend the patch.

It would be great if you send.
Could you resend a patch with aking to Andrew for dropping
Chao's patch?

Thanks!

> 
> 	-ss
> 
> > > 
> > > 
> > > I'm not strongly against and we can proceed with Chao's patch.
> > > 
> > > 	-ss
> > > 
> > > > My point is I'm not saying you're wrong but adding a new stat is easy
> > > > and I need a compelling reason that how it can help users.
> > > > 
> > > > Thanks.
> > > > 
> > > > > 
> > > > > 	-ss
> > > > > 
> > > > > > For it, we should modify zram_free_page has return value.
> > > > > > What do other guys think?
> > > > > > 
> > > > > > > 
> > > > > > > In this patch, we add num_discards to stat discarded pages, and export it to
> > > > > > > sysfs for users.
> > > > > > > 
> > > > > > > * From v1
> > > > > > >  * Update zram document to show num_discards in statistics list.
> > > > > > > 
> > > > > > > * From v2
> > > > > > >  * Update description of this patch with clear goal.
> > > > > > > 
> > > > > > > Signed-off-by: Chao Yu <chao2.yu@samsung.com>
> > > > > > > ---
> > > > > > >  Documentation/ABI/testing/sysfs-block-zram | 10 ++++++++++
> > > > > > >  Documentation/blockdev/zram.txt            |  1 +
> > > > > > >  drivers/block/zram/zram_drv.c              |  3 +++
> > > > > > >  drivers/block/zram/zram_drv.h              |  1 +
> > > > > > >  4 files changed, 15 insertions(+)
> > > > > > > 
> > > > > > > diff --git a/Documentation/ABI/testing/sysfs-block-zram b/Documentation/ABI/testing/sysfs-block-zram
> > > > > > > index 70ec992..fa8936e 100644
> > > > > > > --- a/Documentation/ABI/testing/sysfs-block-zram
> > > > > > > +++ b/Documentation/ABI/testing/sysfs-block-zram
> > > > > > > @@ -57,6 +57,16 @@ Description:
> > > > > > >  		The failed_writes file is read-only and specifies the number of
> > > > > > >  		failed writes happened on this device.
> > > > > > >  
> > > > > > > +
> > > > > > > +What:		/sys/block/zram<id>/num_discards
> > > > > > > +Date:		August 2014
> > > > > > > +Contact:	Chao Yu <chao2.yu@samsung.com>
> > > > > > > +Description:
> > > > > > > +		The num_discards file is read-only and specifies the number of
> > > > > > > +		physical blocks which are discarded by this device. These blocks
> > > > > > > +		are included in discard request which is sended by filesystem as
> > > > > > > +		the blocks are no longer used.
> > > > > > > +
> > > > > > >  What:		/sys/block/zram<id>/max_comp_streams
> > > > > > >  Date:		February 2014
> > > > > > >  Contact:	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
> > > > > > > diff --git a/Documentation/blockdev/zram.txt b/Documentation/blockdev/zram.txt
> > > > > > > index 0595c3f..e50e18b 100644
> > > > > > > --- a/Documentation/blockdev/zram.txt
> > > > > > > +++ b/Documentation/blockdev/zram.txt
> > > > > > > @@ -89,6 +89,7 @@ size of the disk when not in use so a huge zram is wasteful.
> > > > > > >  		num_writes
> > > > > > >  		failed_reads
> > > > > > >  		failed_writes
> > > > > > > +		num_discards
> > > > > > >  		invalid_io
> > > > > > >  		notify_free
> > > > > > >  		zero_pages
> > > > > > > diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> > > > > > > index d00831c..904e7a5 100644
> > > > > > > --- a/drivers/block/zram/zram_drv.c
> > > > > > > +++ b/drivers/block/zram/zram_drv.c
> > > > > > > @@ -606,6 +606,7 @@ static void zram_bio_discard(struct zram *zram, u32 index,
> > > > > > >  		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.num_discards);
> > > > > > >  		index++;
> > > > > > >  		n -= PAGE_SIZE;
> > > > > > >  	}
> > > > > > > @@ -866,6 +867,7 @@ ZRAM_ATTR_RO(num_reads);
> > > > > > >  ZRAM_ATTR_RO(num_writes);
> > > > > > >  ZRAM_ATTR_RO(failed_reads);
> > > > > > >  ZRAM_ATTR_RO(failed_writes);
> > > > > > > +ZRAM_ATTR_RO(num_discards);
> > > > > > >  ZRAM_ATTR_RO(invalid_io);
> > > > > > >  ZRAM_ATTR_RO(notify_free);
> > > > > > >  ZRAM_ATTR_RO(zero_pages);
> > > > > > > @@ -879,6 +881,7 @@ static struct attribute *zram_disk_attrs[] = {
> > > > > > >  	&dev_attr_num_writes.attr,
> > > > > > >  	&dev_attr_failed_reads.attr,
> > > > > > >  	&dev_attr_failed_writes.attr,
> > > > > > > +	&dev_attr_num_discards.attr,
> > > > > > >  	&dev_attr_invalid_io.attr,
> > > > > > >  	&dev_attr_notify_free.attr,
> > > > > > >  	&dev_attr_zero_pages.attr,
> > > > > > > diff --git a/drivers/block/zram/zram_drv.h b/drivers/block/zram/zram_drv.h
> > > > > > > index e0f725c..2994aaf 100644
> > > > > > > --- a/drivers/block/zram/zram_drv.h
> > > > > > > +++ b/drivers/block/zram/zram_drv.h
> > > > > > > @@ -86,6 +86,7 @@ struct zram_stats {
> > > > > > >  	atomic64_t num_writes;	/* --do-- */
> > > > > > >  	atomic64_t failed_reads;	/* can happen when memory is too low */
> > > > > > >  	atomic64_t failed_writes;	/* can happen when memory is too low */
> > > > > > > +	atomic64_t num_discards;	/* no. of discarded pages */
> > > > > > >  	atomic64_t invalid_io;	/* non-page-aligned I/O requests */
> > > > > > >  	atomic64_t notify_free;	/* no. of swap slot free notifications */
> > > > > > >  	atomic64_t zero_pages;		/* no. of zero filled pages */
> > > > > > > -- 
> > > > > > > 2.0.1.474.g72c7794
> > > > > > > 
> > > > > > > 
> > > > > > > --
> > > > > > > To unsubscribe, send a message with 'unsubscribe linux-mm' in
> > > > > > > the body to majordomo@kvack.org.  For more info on Linux MM,
> > > > > > > see: http://www.linux-mm.org/ .
> > > > > > > Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
> > > > > > 
> > > > > > -- 
> > > > > > Kind regards,
> > > > > > Minchan Kim
> > > > > > 
> > > > > 
> > > > > --
> > > > > To unsubscribe, send a message with 'unsubscribe linux-mm' in
> > > > > the body to majordomo@kvack.org.  For more info on Linux MM,
> > > > > see: http://www.linux-mm.org/ .
> > > > > Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
> > > > 
> > > > -- 
> > > > Kind regards,
> > > > Minchan Kim
> > > > 
> > > 
> > > --
> > > To unsubscribe, send a message with 'unsubscribe linux-mm' in
> > > the body to majordomo@kvack.org.  For more info on Linux MM,
> > > see: http://www.linux-mm.org/ .
> > > Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
> > 
> > -- 
> > Kind regards,
> > Minchan Kim
> > 
> 
> --
> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> the body to majordomo@kvack.org.  For more info on Linux MM,
> see: http://www.linux-mm.org/ .
> Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

-- 
Kind regards,
Minchan Kim

Re: [PATCH v3] zram: add num_discards for discarded pages stat

[Sergey Senozhatsky]

Hello,

On (09/05/14 09:35), Minchan Kim wrote:
> Hi Sergey,
> 
> On Fri, Sep 05, 2014 at 12:43:31AM +0900, Sergey Senozhatsky wrote:
> > Hello,
> > 
> > On (09/04/14 11:25), Minchan Kim wrote:
> > > Hello Sergey,
> > > 
> > > First of all, Sorry for late response.
> > > 
> > > On Tue, Aug 26, 2014 at 11:15:43PM +0900, Sergey Senozhatsky wrote:
> > > > Hello,
> > > > 
> > > > On (08/26/14 14:08), Minchan Kim wrote:
> > > > > Hi,
> > > > > 
> > > > > On Mon, Aug 25, 2014 at 08:01:18PM +0900, Sergey Senozhatsky wrote:
> > > > > > Hello,
> > > > > > 
> > > > > > On (08/25/14 09:36), Minchan Kim wrote:
> > > > > > > Hello Chao,
> > > > > > > 
> > > > > > > On Fri, Aug 22, 2014 at 04:21:01PM +0800, Chao Yu wrote:
> > > > > > > > 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.
> > > > > > > 
> > > > > > > My concern is that how much we are able to know the effect of discard
> > > > > > > exactly with your patch.
> > > > > > > 
> > > > > > > The issue I can think of is zram-swap discard.
> > > > > > > Now, zram handles notification from VM to free duplicated copy between
> > > > > > > VM-owned memory and zRAM-owned's one so discarding for zram-swap might
> > > > > > > be pointless overhead but your stat indicates lots of free page discarded
> > > > > > > without real freeing 
> > > > > > 
> > > > > > this is why I've moved stats accounting to the place where actual
> > > > > > zs_free() happens. and, frankly, I still would like to see the number
> > > > > > of zs_free() calls, rather than the number of slot free notifications
> > > > > > and REQ_DISCARD (or separately), because they all end up calling
> > > > > > zs_free(). iow, despite the call path, from the user point of view
> > > > > > they are just zs_free() -- the number of pages that's been freed by
> > > > > > the 3rd party and we had have to deal with that.
> > > > > 
> > > > > My qeustion is that what user can do with the only real freeing count?
> > > > > Could you give me a concret example?
> > > > 
> > > > for !swap device case it's identicall to `num_discarded'.
> > > > for swap device case, it's a bit more complicated (less convenient) if
> > > > we actually can receive both slot free and delayed REQ_DISCARDs.
> > > > 
> > > > > It's a just number of real freeing count so if you were admin, what
> > > > > do you expect from that? That's what I'd like to see in changelog.
> > > > > 
> > > > > > 
> > > > > > > so that user might think "We should keep enable
> > > > > > > swap discard for zRAM because the stat indicates it's really good".
> > > > > > > 
> > > > > > > In summary, wouldn't it better to have two?
> > > > > > > 
> > > > > > > num_discards,
> > > > > > > num_failed_discards?
> > > > > > 
> > > > > > do we actully need this? the only value I can think of (perhaps I'm
> > > > > > missing something) is that we can make sure that we need to support
> > > > > > both slot free and REQ_DISCARDS, or we can leave only REQ_DISCARDS.
> > > > > > is there anything else?
> > > > > 
> > > > > The secnario I imagined with two stat is how REQ_DISCARDS is effective
> > > > > from swap layer. Normally, slot free logic is called in advance
> > > > > when the page is zapped or swap read happens to avoid duplicate copy,
> > > > > so discard request from swap space would be just overhead without
> > > > > any benefit so we might guide zram-swap user don't use "swap -d".
> > > > > Otherwise, as failed_discard ratio is low, it means it would be
> > > > > better to remove swap slot free logic because swap discard works well
> > > > > without slot free hint.(Although I don't think)
> > > > 
> > > > yes, so it looks like it is a developer's stat - to make some
> > > > observations and to come up with some decisions. do we really
> > > > want to put it into release?
> > > 
> > > Agree. I was too specific for my purpose and it couldn't be
> > > a compelling reason to make it export for general purpose.
> > > 
> > > Actually, discard req sent by swap for getting free cluster
> > > shouldn't be success(i,e num_discarded should be zero) because
> > > zram_slot_free_notify will always free the duplicated copy
> > > in advance so user don't have any gain with 'swapon -d'.
> > > 
> > > Now, I agree with you that we shouldn't add more stat without
> > > compelling reason so it would be better to rename notify_free
> > > with discarded and move it in zram_free_page like your patch.
> > > https://lkml.org/lkml/2014/8/21/294
> > > 
> > > I will ask to Andrew to revert Chao's patch and pick your patch
> > > after a few days unless Chao has another opinion.
> > > 
> > 
> > no problem.
> > 
> > I, probably, was not clear enough. one of my objections was that
> > it is really easy to add a new stat file, and surprisingly hard to
> > remove it later, even a temporary one. because it's almost impossible
> > to beat the "someone might use it" argument.
> 
> Almost true but it's not the case for sysfs.
> Documentation/sysfs-rulies.txt says
> 
> "The kernel-exported sysfs exports internal kernel implementation details
> and depends on internal kernel structures and layout. It is agreed upon
> by the kernel developers that the Linux kernel does not provide a stable
> internal API. Therefore, there are aspects of the sysfs interface that
> may not be stable across kernel releases."
> 
> So, we could decide a schedule when we removes and warn to user
> if they try to see that. If any serious issue doesn't appear until
> the deadline, we would remove it then.
> 
> You could read Documentation/ABI/README.

oh, didn't know that. thanks!

> 
> > just a side note, my whole impresion is that some of the stats, that we
> > export, belongs to /proc/diskstats, /sys/block/zramX/stat and frieds.
> > I didn't check but I think that some handy tools like iostat/etc are
> > using these files. though I have no idea how often (if ever) people
> > want to track (or at least see) zram in iostat or similar tools.
> 
> Yeb, I knew it and wanted to clean it up in my spare time but
> patches are welcome!
> 
> > 
> > 
> > 
> > please let me know if I have to resend the patch.
> 
> It would be great if you send.
> Could you resend a patch with aking to Andrew for dropping
> Chao's patch?

ok.

	-ss

> Thanks!
> 
> > 
> > 	-ss
> > 
> > > > 
> > > > 
> > > > I'm not strongly against and we can proceed with Chao's patch.
> > > > 
> > > > 	-ss
> > > > 
> > > > > My point is I'm not saying you're wrong but adding a new stat is easy
> > > > > and I need a compelling reason that how it can help users.
> > > > > 
> > > > > Thanks.
> > > > > 
> > > > > > 
> > > > > > 	-ss
> > > > > > 
> > > > > > > For it, we should modify zram_free_page has return value.
> > > > > > > What do other guys think?
> > > > > > > 
> > > > > > > > 
> > > > > > > > In this patch, we add num_discards to stat discarded pages, and export it to
> > > > > > > > sysfs for users.
> > > > > > > > 
> > > > > > > > * From v1
> > > > > > > >  * Update zram document to show num_discards in statistics list.
> > > > > > > > 
> > > > > > > > * From v2
> > > > > > > >  * Update description of this patch with clear goal.
> > > > > > > > 
> > > > > > > > Signed-off-by: Chao Yu <chao2.yu@samsung.com>
> > > > > > > > ---
> > > > > > > >  Documentation/ABI/testing/sysfs-block-zram | 10 ++++++++++
> > > > > > > >  Documentation/blockdev/zram.txt            |  1 +
> > > > > > > >  drivers/block/zram/zram_drv.c              |  3 +++
> > > > > > > >  drivers/block/zram/zram_drv.h              |  1 +
> > > > > > > >  4 files changed, 15 insertions(+)
> > > > > > > > 
> > > > > > > > diff --git a/Documentation/ABI/testing/sysfs-block-zram b/Documentation/ABI/testing/sysfs-block-zram
> > > > > > > > index 70ec992..fa8936e 100644
> > > > > > > > --- a/Documentation/ABI/testing/sysfs-block-zram
> > > > > > > > +++ b/Documentation/ABI/testing/sysfs-block-zram
> > > > > > > > @@ -57,6 +57,16 @@ Description:
> > > > > > > >  		The failed_writes file is read-only and specifies the number of
> > > > > > > >  		failed writes happened on this device.
> > > > > > > >  
> > > > > > > > +
> > > > > > > > +What:		/sys/block/zram<id>/num_discards
> > > > > > > > +Date:		August 2014
> > > > > > > > +Contact:	Chao Yu <chao2.yu@samsung.com>
> > > > > > > > +Description:
> > > > > > > > +		The num_discards file is read-only and specifies the number of
> > > > > > > > +		physical blocks which are discarded by this device. These blocks
> > > > > > > > +		are included in discard request which is sended by filesystem as
> > > > > > > > +		the blocks are no longer used.
> > > > > > > > +
> > > > > > > >  What:		/sys/block/zram<id>/max_comp_streams
> > > > > > > >  Date:		February 2014
> > > > > > > >  Contact:	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
> > > > > > > > diff --git a/Documentation/blockdev/zram.txt b/Documentation/blockdev/zram.txt
> > > > > > > > index 0595c3f..e50e18b 100644
> > > > > > > > --- a/Documentation/blockdev/zram.txt
> > > > > > > > +++ b/Documentation/blockdev/zram.txt
> > > > > > > > @@ -89,6 +89,7 @@ size of the disk when not in use so a huge zram is wasteful.
> > > > > > > >  		num_writes
> > > > > > > >  		failed_reads
> > > > > > > >  		failed_writes
> > > > > > > > +		num_discards
> > > > > > > >  		invalid_io
> > > > > > > >  		notify_free
> > > > > > > >  		zero_pages
> > > > > > > > diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> > > > > > > > index d00831c..904e7a5 100644
> > > > > > > > --- a/drivers/block/zram/zram_drv.c
> > > > > > > > +++ b/drivers/block/zram/zram_drv.c
> > > > > > > > @@ -606,6 +606,7 @@ static void zram_bio_discard(struct zram *zram, u32 index,
> > > > > > > >  		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.num_discards);
> > > > > > > >  		index++;
> > > > > > > >  		n -= PAGE_SIZE;
> > > > > > > >  	}
> > > > > > > > @@ -866,6 +867,7 @@ ZRAM_ATTR_RO(num_reads);
> > > > > > > >  ZRAM_ATTR_RO(num_writes);
> > > > > > > >  ZRAM_ATTR_RO(failed_reads);
> > > > > > > >  ZRAM_ATTR_RO(failed_writes);
> > > > > > > > +ZRAM_ATTR_RO(num_discards);
> > > > > > > >  ZRAM_ATTR_RO(invalid_io);
> > > > > > > >  ZRAM_ATTR_RO(notify_free);
> > > > > > > >  ZRAM_ATTR_RO(zero_pages);
> > > > > > > > @@ -879,6 +881,7 @@ static struct attribute *zram_disk_attrs[] = {
> > > > > > > >  	&dev_attr_num_writes.attr,
> > > > > > > >  	&dev_attr_failed_reads.attr,
> > > > > > > >  	&dev_attr_failed_writes.attr,
> > > > > > > > +	&dev_attr_num_discards.attr,
> > > > > > > >  	&dev_attr_invalid_io.attr,
> > > > > > > >  	&dev_attr_notify_free.attr,
> > > > > > > >  	&dev_attr_zero_pages.attr,
> > > > > > > > diff --git a/drivers/block/zram/zram_drv.h b/drivers/block/zram/zram_drv.h
> > > > > > > > index e0f725c..2994aaf 100644
> > > > > > > > --- a/drivers/block/zram/zram_drv.h
> > > > > > > > +++ b/drivers/block/zram/zram_drv.h
> > > > > > > > @@ -86,6 +86,7 @@ struct zram_stats {
> > > > > > > >  	atomic64_t num_writes;	/* --do-- */
> > > > > > > >  	atomic64_t failed_reads;	/* can happen when memory is too low */
> > > > > > > >  	atomic64_t failed_writes;	/* can happen when memory is too low */
> > > > > > > > +	atomic64_t num_discards;	/* no. of discarded pages */
> > > > > > > >  	atomic64_t invalid_io;	/* non-page-aligned I/O requests */
> > > > > > > >  	atomic64_t notify_free;	/* no. of swap slot free notifications */
> > > > > > > >  	atomic64_t zero_pages;		/* no. of zero filled pages */
> > > > > > > > -- 
> > > > > > > > 2.0.1.474.g72c7794
> > > > > > > > 
> > > > > > > > 
> > > > > > > > --
> > > > > > > > To unsubscribe, send a message with 'unsubscribe linux-mm' in
> > > > > > > > the body to majordomo@kvack.org.  For more info on Linux MM,
> > > > > > > > see: http://www.linux-mm.org/ .
> > > > > > > > Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
> > > > > > > 
> > > > > > > -- 
> > > > > > > Kind regards,
> > > > > > > Minchan Kim
> > > > > > > 
> > > > > > 
> > > > > > --
> > > > > > To unsubscribe, send a message with 'unsubscribe linux-mm' in
> > > > > > the body to majordomo@kvack.org.  For more info on Linux MM,
> > > > > > see: http://www.linux-mm.org/ .
> > > > > > Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
> > > > > 
> > > > > -- 
> > > > > Kind regards,
> > > > > Minchan Kim
> > > > > 
> > > > 
> > > > --
> > > > To unsubscribe, send a message with 'unsubscribe linux-mm' in
> > > > the body to majordomo@kvack.org.  For more info on Linux MM,
> > > > see: http://www.linux-mm.org/ .
> > > > Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
> > > 
> > > -- 
> > > Kind regards,
> > > Minchan Kim
> > > 
> > 
> > --
> > To unsubscribe, send a message with 'unsubscribe linux-mm' in
> > the body to majordomo@kvack.org.  For more info on Linux MM,
> > see: http://www.linux-mm.org/ .
> > Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
> 
> -- 
> Kind regards,
> Minchan Kim
>