linux-kernel - Re: [PATCH v4 2/4] zram: implement deduplication in zram

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Date:   Wed, 26 Apr 2017 15:29:38 +0900
From:   Minchan Kim <minchan@...nel.org>
To:     Joonsoo Kim <js1304@...il.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        linux-kernel@...r.kernel.org, kernel-team@....com
Subject: Re: [PATCH v4 2/4] zram: implement deduplication in zram

Hi Sergey and Joonsoo,

On Wed, Apr 26, 2017 at 02:57:03PM +0900, Joonsoo Kim wrote:
> On Wed, Apr 26, 2017 at 11:14:52AM +0900, Sergey Senozhatsky wrote:
> > Hello,
> > 
> > On (04/26/17 09:52), js1304@...il.com wrote:
> > [..]
> > >  	ret = scnprintf(buf, PAGE_SIZE,
> > > -			"%8llu %8llu %8llu %8lu %8ld %8llu %8lu\n",
> > > +			"%8llu %8llu %8llu %8lu %8ld %8llu %8lu %8llu %8llu\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.same_pages),
> > > -			pool_stats.pages_compacted);
> > > +			pool_stats.pages_compacted,
> > > +			zram_dedup_dup_size(zram),
> > > +			zram_dedup_meta_size(zram));
> > 
> > hm... should't we subtract zram_dedup_dup_size(zram) from
> > ->stats.compr_data_size? we don't use extra memory for dedupped
> > pages. or don't inc ->stats.compr_data_size for dedupped pages?
> 
> Hmm... My intention is to keep previous stat as much as possible. User
> can just notice the saving by only checking mem_used.
> 
> However, it's also odd that compr_data_size doesn't show actual
> compressed data size.

Actually, I found it for the last review cycle but didn't say that
intentionally. Because it is also odd to me that pages_stored isn't
increased for same_pages so I thought we can fix it all.

I mean:

* normal page
        inc pages_stored
        inc compr_data_size
* same_page
        inc pages_stored
        inc same_pages
* dedup_page
        inc pages_stored
        inc dup_data_size
         
IOW, pages_stored should be increased for every write IO.
But the concern is we have said in zram.txt

 orig_data_size   uncompressed size of data stored in this disk.
                  This excludes same-element-filled pages (same_pages) since
                  no memory is allocated for them.

So, we might be too late. :-(
What do you think about it?
If anyone doesn't have any objection, I want to correct it all.

Thanks.