Date:	Mon, 19 Aug 2013 13:37:58 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Bob Liu <bob.liu@...cle.com>
Cc:	Mel Gorman <mgorman@...e.de>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jens Axboe <axboe@...nel.dk>,
	Seth Jennings <sjenning@...ux.vnet.ibm.com>,
	Nitin Gupta <ngupta@...are.org>,
	Konrad Rzeszutek Wilk <konrad@...nok.org>,
	Luigi Semenzato <semenzato@...gle.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Pekka Enberg <penberg@...helsinki.fi>
Subject: Re: [PATCH v6 0/5] zram/zsmalloc promotion

Hello Bob,

Sorry for the late response. I was on holiday.

On Mon, Aug 19, 2013 at 11:57:41AM +0800, Bob Liu wrote:
> Hi Minchan,
> 
> On 08/19/2013 11:18 AM, Minchan Kim wrote:
> > Hello Mel,
> > 
> > On Fri, Aug 16, 2013 at 09:33:47AM +0100, Mel Gorman wrote:
> >> On Fri, Aug 16, 2013 at 01:26:41PM +0900, Minchan Kim wrote:
> >>>>>> <SNIP>
> >>>>>> If it's used for something like tmpfs then it becomes much worse. Normal
> >>>>>> tmpfs without swap can lockup if tmpfs is allowed to fill memory. In a
> >>>>>> sane configuration, lockups will be avoided and deleting a tmpfs file is
> >>>>>> guaranteed to free memory. When zram is used to back tmpfs, there is no
> >>>>>> guarantee that any memory is freed due to fragmentation of the compressed
> >>>>>> pages. The only way to recover the memory may be to kill applications
> >>>>>> holding tmpfs files open and then delete them which is fairly drastic
> >>>>>> action in a normal server environment.
> >>>>>
> >>>>> Indeed.
> >>>>> Actually, I had a plan to support zsmalloc compaction. The zsmalloc exposes
> >>>>> handle instead of pure pointer so it could migrate some zpages to somewhere
> >>>>> to pack in. Then, it could help above problem and OOM storm problem.
> >>>>> Anyway, it's a totally new feature and requires many changes and experiement.
> >>>>> Although we don't have such feature, zram is still good for many people.
> >>>>>
> >>>>
> >>>> And is zsmalloc was pluggable for zswap then it would also benefit.
> >>>
> >>> But zswap isn't pseudo block device so it couldn't be used for block device.
> >>
> >> It would not be impossible to write one. Taking a quick look it might even
> >> be doable by just providing a zbud_ops that does not have an evict handler
> >> and make sure the errors are handled correctly. i.e. does the following
> >> patch mean that zswap never writes back and instead just compresses pages
> >> in memory?
> >>
> >> diff --git a/mm/zswap.c b/mm/zswap.c
> >> index deda2b6..99e41c8 100644
> >> --- a/mm/zswap.c
> >> +++ b/mm/zswap.c
> >> @@ -819,7 +819,6 @@ static void zswap_frontswap_invalidate_area(unsigned type)
> >>  }
> >>  
> >>  static struct zbud_ops zswap_zbud_ops = {
> >> -	.evict = zswap_writeback_entry
> >>  };
> >>  
> >>  static void zswap_frontswap_init(unsigned type)
> >>
> >> If so, it should be doable to link that up in a sane way so it can be
> >> configured at runtime.
> >>
> >> Did you ever even try something like this?
> > 
> > Never. Because I didn't have such requirement for zram.
> > 
> >>
> >>> Let say one usecase for using zram-blk.
> >>>
> >>> 1) Many embedded system don't have swap so although tmpfs can support swapout
> >>> it's pointless still so such systems should have sane configuration to limit
> >>> memory space so it's not only zram problem.
> >>>
> >>
> >> If zswap was backed by a pseudo device that failed all writes or an an
> >> ops with no evict handler then it would be functionally similar.
> >>
> >>> 2) Many embedded system don't have enough memory. Let's assume short-lived
> >>> file growing up until half of system memory once in a while. We don't want
> >>> to write it on flash by wear-leveing issue and very slowness so we want to use
> >>> in-memory but if we uses tmpfs, it should evict half of working set to cover
> >>> them when the size reach peak. zram would be better choice.
> >>>
> >>
> >> Then back it by a pseudo device that fails all writes so it does not have
> >> to write to disk.
> > 
> > You mean "make pseudo block device and register make_request_fn
> > and prevent writeback". Bah, yes, it's doable but what is it different with below?
> > 
> > 1) move zbud into zram
> > 2) implement frontswap API in zram
> > 3) implement writebazk in zram
> > 
> > The zram has been for a long time in staging to be promoted and have been
> > maintained/deployed. Of course, I have asked the promotion several times
> > for above a year.
> > 
> > Why can't zram include zswap functions if you really want to merge them?
> > Is there any problem?
> 
> I think merging zram into zswap or merging zswap into zram are the same
> thing. It's no difference.

True but i'd like to merge zswap code into zram.
Because as you know, zram has already lots of users while zswap is almost
new young so I'd like to keep backward compatibility for zram so moving zswap code
into zram is more handy and could keep the git log as well.

> Both way will result in a solution finally with zram block device,
> frontswap API etc.

Right but z* family people should discuss that zswap-writeback is really
good solution for compressed swap. Firstly, I thought zswap is differnt with
zram so there is no issue to promote zram so I and Nitin helped zsmalloc
promotion for Seth and have reviewed at zswap inital phases but the situation
is chainging. Let's discussion further points about compresssed swap solution.
I raised issues as reply of Mel in my thread. Let's think of it.

> 
> The difference is just the name and the merging patch title, I think
> it's unimportant.

If we decide merging them, yes, module name would be important and
we can't ignore copyright and maintainer part, either. Anyway,
I'd like to go that way as last resort afther enough thinking which can justify
frontswap-based swap writeback is right approach.

> 
> I've implemented a series [PATCH 0/4] mm: merge zram into zswap, I can
> change the tile to "merge zswap into zram" if you want and rename zswap
> to something like zhybrid.

Hmm, I saw that roughly and you are ignoring zram's backward compatibility
and zram-blk functionality which can be used for in-memory compressed block
device without swap.

Thanks.

-- 
Kind regards,
Minchan Kim
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