linux-kernel - Re: [PATCH] mm: extend zero pages to same element pages for zram

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Date:   Fri, 13 Jan 2017 15:23:43 +0900
From:   Minchan Kim <minchan@...nel.org>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     zhouxianrong@...wei.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
        sergey.senozhatsky@...il.com, ngupta@...are.org,
        Mi.Sophia.Wang@...wei.com, zhouxiyu@...wei.com,
        weidu.du@...wei.com, zhangshiming5@...wei.com,
        won.ho.park@...wei.com
Subject: Re: [PATCH] mm: extend zero pages to same element pages for zram

Hi Sergey,

On Fri, Jan 13, 2017 at 01:24:44PM +0900, Sergey Senozhatsky wrote:
> Hello,
> 
> sorry, was mostly offline for the past few days, now catching up.
> 
> On (01/10/17 08:41), Minchan Kim wrote:
> > > the idea is that without doing more calculations we extend zero pages
> > > to same element pages for zram. zero page is special case of
> > > same element page with zero element.
> > > 
> 
> interesting idea.
> 
> [..]
> > >  	flush_dcache_page(page);
> > > @@ -431,7 +479,7 @@ static ssize_t mm_stat_show(struct device *dev,
> > >  			mem_used << PAGE_SHIFT,
> > >  			zram->limit_pages << PAGE_SHIFT,
> > >  			max_used << PAGE_SHIFT,
> > > -			(u64)atomic64_read(&zram->stats.zero_pages),
> > > +			(u64)atomic64_read(&zram->stats.same_pages),
> > 
> > Unfortunately, we cannot replace zero pages stat with same pages's one right
> > now due to compatibility problem. Please add same_pages to tail of the stat
> > and we should warn deprecated zero_pages stat so we finally will remove it
> > two year later. Please reference Documentation/ABI/obsolete/sysfs-block-zram
> > And add zero-pages to the document.
> > 
> > For example,
> > 
> > ... mm_stat_show()
> > {
> >         pr_warn_once("zero pages was deprecated so it will be removed at 2019 Jan");
> > }
> > 
> > Sergey, what's your opinion?
> 
> oh, I was going to ask you whether you have any work in progress at
> the moment or not. because deprecated attrs are scheduled to be removed
> in 4.11. IOW, we must send the clean up patch, well, right now. so I can
> prepare the patch, but it can conflict with someone's 'more serious/relevant'
> work.

I think deprecating attrs is top priority to me so go ahead. :)

> 
> we also have zram hot/addd sysfs attr, which must be deprecated and
> converted to a char device. per Greg KH.
> 
> > Please add same_pages to tail of the stat
> 
> sounds ok to me. and yes, can deprecate zero_pages.
> 
> seems that with that patch the concept of ZRAM_ZERO disappears. both
> ZERO and SAME_ELEMENT pages are considered to be the same thing now.

Right.

> which is fine and makes sense to me, I think. and if ->.same_pages will
> replace ->.zero_pages in mm_stat() then I'm also OK. yes, we will see
> increased number in the last column of mm_stat file, but I don't tend
> to see any issues here. Minchan, what do you think?

Could you elaborate a bit? Do you mean this?

        ret = scnprintf(buf, PAGE_SIZE,
                        "%8llu %8llu %8llu %8lu %8ld %8llu %8lu\n",
                        orig_size << PAGE_SHIFT,
                        (u64)atomic64_read(&zram->stats.compr_data_size),
                        mem_used << PAGE_SHIFT,
                        zram->limit_pages << PAGE_SHIFT,
                        max_used << PAGE_SHIFT,
                        // (u64)atomic64_read(&zram->stats.zero_pages),
                        (u64)atomic64_read(&zram->stats.same_pages),
                        pool_stats.pages_compacted);