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

From: Minchan Kim
Date: Wed Aug 20 2014 - 21:18:25 EST

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Hi Chao,

On Wed, Aug 20, 2014 at 04:20:48PM +0800, Chao Yu wrote:
> Hi Minchan,
>
> > -----Original Message-----
> > From: Minchan Kim [mailto:minchan@xxxxxxxxxx]
> > Sent: Wednesday, August 20, 2014 10:09 AM
> > To: Sergey Senozhatsky
> > Cc: Chao Yu; linux-kernel@xxxxxxxxxxxxxxx; linux-mm@xxxxxxxxx; ngupta@xxxxxxxxxx; 'Jerome
> > Marchand'; 'Andrew Morton'
> > Subject: Re: [PATCH] zram: add num_discards for discarded pages stat
> >
> > Hi Sergey,
> >
> > On Tue, Aug 19, 2014 at 08:25:00PM +0900, Sergey Senozhatsky wrote:
> > > Hello,
> > >
> > > On (08/19/14 13:45), Chao Yu wrote:
> > > > > On (08/15/14 11:27), Chao Yu wrote:
> > > > > > Now we have supported handling discard request which is sended by filesystem,
> > > > > > but no interface could be used to show information of discard.
> > > > > > This patch adds num_discards to stat discarded pages, then export it to sysfs
> > > > > > for displaying.
> > > > > >
> > > > >
> > > > > a side question: we account discarded pages via slot free notify in
> > > > > notify_free and via req_discard in num_discards. how about accounting
> > > > > both of them in num_discards? because, after all, they account a number
> > > > > of discarded pages (zram_free_page()). or there any particular reason we
> > > > > want to distinguish.
> > > >
> > > > Yeah, I agree with you as I have no such reason unless there are our users'
> > > > explicitly requirement for showing notify_free/num_discards separately later.
> > > >
> > > > How do you think of sending another patch to merge these two counts?
> > > >
> > >
> > > Minchan, what do you think? let's account discarded pages in one place.
> >
> > First of all, I'd like to know why we need num_discards.
> > It should be in description and depends on it whether we should merge both
> > counts or separate.
>
> Oh, it's my mistaken.
>
> When commit 9b9913d80b2896ecd9e0a1a8f167ccad66fac79c (Staging: zram: Update
> zram documentation) and commit e98419c23b1a189c932775f7833e94cb5230a16b (Staging:
> zram: Document sysfs entries) description related to 'discard' stat was designed
> and added to zram.txt and sysfs-block-zram, but without implementation of function
> for handling discard request, description in documents were removed in commit
> 8dd1d3247e6c00b50ef83934ea8b22a1590015de (zram: document failed_reads,
> failed_writes stats)

Thanks for letting me know the history.

>
> For now, we have already supported discard handling, so it's better to resume
> the stat of discard number, this discard stat supports user one more kind of runtime
> information of zram as other stats supported.
>
> How do you think?

I'm not strong against the idea but just "resume is better" and
"one more is problem as other stats supported" is not logical
to me.

You should explain why we need such new stat so that user can take
what kinds of benefit from that. Otherwise, we couldn't know the stat
is best or not for the goal.

I might be paranoid about small stuff and I admit I'm not good for it,
too but pz, understand that adding the new feature requires a
good description which should include clear goal.

I hope I'm not discouraging. :)

>
> >
> > Thanks.
> >
> >
> >
> > >
> > > > One more thing is that I am missing to update document of zram, sorry about
> > > > that, let me update it in v2.
> > >
> > > thanks.
> > >
> > > -ss
> > >
> > > > Thanks,
> > > > Yu
> > > >
> > > > >
> > > > > -ss
> > > > >
> > > > > > Signed-off-by: Chao Yu <chao2.yu@xxxxxxxxxxx>
> > > > > > ---
> > > > > > Documentation/ABI/testing/sysfs-block-zram | 10 ++++++++++
> > > > > > drivers/block/zram/zram_drv.c | 3 +++
> > > > > > drivers/block/zram/zram_drv.h | 1 +
> > > > > > 3 files changed, 14 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@xxxxxxxxxxx>
> > > > > > +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@xxxxxxxxx>
> > > > > > 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@xxxxxxxxxx For more info on Linux MM,
> > > > > see: http://www.linux-mm.org/ .
> > > > > Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
> > > >
> > >
> > > --
> > > To unsubscribe, send a message with 'unsubscribe linux-mm' in
> > > the body to majordomo@xxxxxxxxxx For more info on Linux MM,
> > > see: http://www.linux-mm.org/ .
> > > Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
> >
> > --
> > Kind regards,
> > Minchan Kim
>
> --
> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> the body to majordomo@xxxxxxxxxx For more info on Linux MM,
> see: http://www.linux-mm.org/ .
> Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>

--
Kind regards,
Minchan Kim
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Next message: Benjamin Poirier: "Re: [PATCH 2/3] tg3: Fix tx_pending checks for tg3_tso_bug"
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In reply to: Chao Yu: "RE: [PATCH] zram: add num_discards for discarded pages stat"
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