linux-kernel - Re: [PATCH] driver: zram: fix wrong counting on pages

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Date:   Tue, 27 Jul 2021 13:45:04 +0800
From:   Zhaoyang Huang <huangzhaoyang@...il.com>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Zhaoyang Huang <zhaoyang.huang@...soc.com>,
        "open list:MEMORY MANAGEMENT" <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Nitin Gupta <ngupta@...are.org>
Subject: Re: [PATCH] driver: zram: fix wrong counting on pages_store

On Tue, Jul 27, 2021 at 9:32 AM Zhaoyang Huang <huangzhaoyang@...il.com> wrote:
>
> On Tue, Jul 27, 2021 at 1:53 AM Minchan Kim <minchan@...nel.org> wrote:
> >
> > On Mon, Jul 26, 2021 at 10:27:40AM +0900, Sergey Senozhatsky wrote:
> > > On (21/07/20 14:47), Huangzhaoyang wrote:
> > > > It makes the statistic confused when counting pages_store in during writeback.
> > > > pages_store should just reflect the swapped pages on ZRAM, while bd_writes be
> > > > responsible for writeback things.
> > > >
> > > > Signed-off-by: Zhaoyang Huang <zhaoyang.huang@...soc.com>
> > > > ---
> > > >  drivers/block/zram/zram_drv.c | 1 -
> > > >  1 file changed, 1 deletion(-)
> > > >
> > > > diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> > > > index cf8deec..6bdaa9d 100644
> > > > --- a/drivers/block/zram/zram_drv.c
> > > > +++ b/drivers/block/zram/zram_drv.c
> > > > @@ -765,7 +765,6 @@ static ssize_t writeback_store(struct device *dev,
> > > >             zram_set_flag(zram, index, ZRAM_WB);
> > > >             zram_set_element(zram, index, blk_idx);
> > > >             blk_idx = 0;
> > > > -           atomic64_inc(&zram->stats.pages_stored);
> > >
> > > Yeah, I think we don't need it here.
> > >
> > > Minchan, what do you think?
> >
> > What pages_stored currently represents is orig_data_size from mm_stat
> >
> >  ================ =============================================================
> >  orig_data_size   uncompressed size of data stored in this disk.
> >                   Unit: bytes
> >
> > Thus, if we don't increase it after zram_free_page, it loses what's
> > the original data size which would be important to calculate
> > compression ratio along with compr_data_size.
> zram_free_page??  What the commit try to fix is pages_stored being
> increased incorrectly in writeback_store, which will lead to wrong
> statistic on orig size of ZRAM.
OK, if I understand right, I think what you mean is the page count
corresponding to SWAP should be still available even if it is written
back to the backing dev.So we need to increase page_stored again after
zram_free_page free it.