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

From: Minchan Kim
Date: Tue Apr 09 2013 - 19:40:11 EST

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@xxxxxxxxxx> 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. :)


Kind regards,
Minchan Kim
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