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Date: Tue, 16 Sep 2014 00:00:01 +0900
From: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To: Minchan Kim <minchan@...nel.org>
Cc: Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Jerome Marchand <jmarchan@...hat.com>,
Nitin Gupta <ngupta@...are.org>,
Chao Yu <chao2.yu@...sung.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCHv2] zram: use notify_free to account all free notifications
Hi Minchan,
On (09/15/14 08:22), Minchan Kim wrote:
> Hi Sergey,
>
> On Sat, Sep 13, 2014 at 12:52:14PM +0900, Sergey Senozhatsky wrote:
> > notify_free device attribute accounts the number of slot free notifications
> > and internally represents the number of zram_free_page() calls. Slot free
> > notifications are sent only when device is used as a swap device, hence
> > notify_free is used only for swap devices. Since f4659d8e620d08 (zram:
> > support REQ_DISCARD) ZRAM handles yet another one free notification (also
> > via zram_free_page() call) -- REQ_DISCARD requests, which are sent by a
> > filesystem, whenever some data blocks are discarded. However, there is no
> > way to know the number of notifications in the latter case.
> >
> > Change zram_free_page() to return a bool status, indicating if zs_free()
> > has happened. So we can use `notify_free' to account the number of pages
> > freed by zram_bio_discard() and zram_slot_free_notify().
> >
> > This means that depending on usage scenario `notify_free' represents:
> > a) the number of pages freed because of slot free notifications, which is
> > equal to the number of swap_slot_free_notify() calls, so there is no
> > behaviour change
> >
> > b) the number of pages freed because of REQ_DISCARD notifications
> >
> > Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
> > ---
> > Documentation/ABI/testing/sysfs-block-zram | 13 ++++++++-----
> > drivers/block/zram/zram_drv.c | 12 +++++++-----
> > 2 files changed, 15 insertions(+), 10 deletions(-)
> >
> > diff --git a/Documentation/ABI/testing/sysfs-block-zram b/Documentation/ABI/testing/sysfs-block-zram
> > index b13dc99..a6148ea 100644
> > --- a/Documentation/ABI/testing/sysfs-block-zram
> > +++ b/Documentation/ABI/testing/sysfs-block-zram
> > @@ -77,11 +77,14 @@ What: /sys/block/zram<id>/notify_free
> > Date: August 2010
> > Contact: Nitin Gupta <ngupta@...are.org>
> > Description:
> > - The notify_free file is read-only and specifies the number of
> > - swap slot free notifications received by this device. These
> > - notifications are sent to a swap block device when a swap slot
> > - is freed. This statistic is applicable only when this disk is
> > - being used as a swap disk.
> > + The notify_free file is read-only. Depending on device usage
> > + scenario it may account a) the number of pages freed because
> > + of swap slot free notifications or b) the number of pages freed
> > + because of REQ_DISCARD requests sent by bio. The former ones
> > + are sent to a swap block device when a swap slot is freed, which
> > + implies that this disk is being used as a swap disk. The latter
> > + ones are sent by filesystem mounted with discard option,
> > + whenever some data blocks are getting discarded.
> >
> > What: /sys/block/zram<id>/zero_pages
> > Date: August 2010
> > diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> > index d78b245..03d11d5 100644
> > --- a/drivers/block/zram/zram_drv.c
> > +++ b/drivers/block/zram/zram_drv.c
> > @@ -388,7 +388,7 @@ static void handle_zero_page(struct bio_vec *bvec)
> > * caller should hold this table index entry's bit_spinlock to
> > * indicate this index entry is accessing.
> > */
> > -static void zram_free_page(struct zram *zram, size_t index)
> > +static bool zram_free_page(struct zram *zram, size_t index)
> > {
> > struct zram_meta *meta = zram->meta;
> > unsigned long handle = meta->table[index].handle;
> > @@ -402,7 +402,7 @@ static void zram_free_page(struct zram *zram, size_t index)
> > zram_clear_flag(meta, index, ZRAM_ZERO);
> > atomic64_dec(&zram->stats.zero_pages);
> > }
> > - return;
> > + return false;
> > }
> >
> > zs_free(meta->mem_pool, handle);
> > @@ -413,6 +413,7 @@ static void zram_free_page(struct zram *zram, size_t index)
> >
> > meta->table[index].handle = 0;
> > zram_set_obj_size(meta, index, 0);
> > + return true;
> > }
>
> For just stat accounting, adding return value for fast path?
> I don't think it's not a important stat at the cost of adding more overhead
> in fastpath. If you have a strong reason, I will do that. Otherwise,
> please, don't touch fast path and just account it regardless of real freeing.
yes, that was the reason -- we can show just a number of notifications
or a number of notifications that forced zram to take some actions (free memory).
my target was the second case. [[besides, a single mov could not dramatically
impact the perfomance; we do much heavier staff when we actually free the
memory]].
> I should have said before resending.
> Sorry for bothering you. :)
no problem, resent v3.
hoping this time it'll work out for everyone :)
-ss
>
> >
> > static int zram_decompress_page(struct zram *zram, char *mem, u32 index)
> > @@ -696,7 +697,8 @@ static void zram_bio_discard(struct zram *zram, u32 index,
> >
> > while (n >= PAGE_SIZE) {
> > bit_spin_lock(ZRAM_ACCESS, &meta->table[index].value);
> > - zram_free_page(zram, index);
> > + if (zram_free_page(zram, index))
> > + atomic64_inc(&zram->stats.notify_free);
> > bit_spin_unlock(ZRAM_ACCESS, &meta->table[index].value);
> > index++;
> > n -= PAGE_SIZE;
> > @@ -936,9 +938,9 @@ static void zram_slot_free_notify(struct block_device *bdev,
> > meta = zram->meta;
> >
> > bit_spin_lock(ZRAM_ACCESS, &meta->table[index].value);
> > - zram_free_page(zram, index);
> > + if (zram_free_page(zram, index))
> > + atomic64_inc(&zram->stats.notify_free);
> > bit_spin_unlock(ZRAM_ACCESS, &meta->table[index].value);
> > - atomic64_inc(&zram->stats.notify_free);
> > }
> >
> > static const struct block_device_operations zram_devops = {
> > --
> > 2.1.0.251.ga182987
> >
> > --
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> > Please read the FAQ at http://www.tux.org/lkml/
>
> --
> Kind regards,
> Minchan Kim
>
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