linux-kernel - Re: [PATCH] mm: remove compressed copy from zram in-memory

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Date:	Wed, 10 Apr 2013 08:40:02 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Ric Mason <ric.masonn@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Hugh Dickins <hughd@...gle.com>,
	Seth Jennings <sjenning@...ux.vnet.ibm.com>,
	Nitin Gupta <ngupta@...are.org>,
	Konrad Rzeszutek Wilk <konrad@...nok.org>,
	Shaohua Li <shli@...nel.org>,
	Dan Magenheimer <dan.magenheimer@...cle.com>
Subject: Re: [PATCH] mm: remove compressed copy from zram in-memory

On Tue, Apr 09, 2013 at 01:36:52PM +0800, Ric Mason wrote:
> Hi Minchan,
> On 04/09/2013 09:02 AM, Minchan Kim wrote:
> >Hi Andrew,
> >
> >On Mon, Apr 08, 2013 at 02:17:10PM -0700, Andrew Morton wrote:
> >>On Mon,  8 Apr 2013 15:01:02 +0900 Minchan Kim <minchan@...nel.org> wrote:
> >>
> >>>Swap subsystem does lazy swap slot free with expecting the page
> >>>would be swapped out again so we can avoid unnecessary write.
> >>Is that correct?  How can it save a write?
> >Correct.
> >
> >The add_to_swap makes the page dirty and we must pageout only if the page is
> >dirty. If a anon page is already charged into swapcache, we skip writeout
> >the page in shrink_page_list, then just remove the page from swapcache and
> >free it by __remove_mapping.
> >
> >I did received same question multiple time so it would be good idea to
> >write down it in vmscan.c somewhere.
> >
> >>>But the problem in in-memory swap(ex, zram) is that it consumes
> >>>memory space until vm_swap_full(ie, used half of all of swap device)
> >>>condition meet. It could be bad if we use multiple swap device,
> >>>small in-memory swap and big storage swap or in-memory swap alone.
> >>>
> >>>This patch makes swap subsystem free swap slot as soon as swap-read
> >>>is completed and make the swapcache page dirty so the page should
> >>>be written out the swap device to reclaim it.
> >>>It means we never lose it.
> >>>From my reading of the patch, that isn't how it works?  It changed
> >>end_swap_bio_read() to call zram_slot_free_notify(), which appears to
> >>free the underlying compressed page.  I have a feeling I'm hopelessly
> >>confused.
> >You understand right totally.
> >Selecting swap slot in my description was totally miss.
> >Need to rewrite the description.
> 
> free the swap slot and free compress page is the same, isn't it?

I think so.
I just wanted to make my description more clear with more general terms. :)

Thanks.

-- 
Kind regards,
Minchan Kim
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