Date:   Thu, 11 May 2017 11:13:04 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Joonsoo Kim <js1304@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Rik van Riel <riel@...hat.com>,
        Johannes Weiner <hannes@...xchg.org>,
        mgorman@...hsingularity.net, Laura Abbott <lauraa@...eaurora.org>,
        Minchan Kim <minchan@...nel.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Michal Nazarewicz <mina86@...a86.com>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Russell King <linux@...linux.org.uk>,
        Will Deacon <will.deacon@....com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kernel-team@....com
Subject: Re: [PATCH v7 0/7] Introduce ZONE_CMA

On Thu 11-05-17 11:12:43, Joonsoo Kim wrote:
> Sorry for the late response. I was on a vacation.
> 
> On Tue, May 02, 2017 at 03:32:29PM +0200, Michal Hocko wrote:
> > On Tue 02-05-17 13:01:32, Joonsoo Kim wrote:
> > > On Thu, Apr 27, 2017 at 05:06:36PM +0200, Michal Hocko wrote:
> > [...]
> > > > I see this point and I agree that using a specific zone might be a
> > > > _nicer_ solution in the end but you have to consider another aspects as
> > > > well. The main one I am worried about is a long term maintainability.
> > > > We are really out of page flags and consuming one for a rather specific
> > > > usecase is not good. Look at ZONE_DMA. I am pretty sure that almost
> > > > no sane HW needs 16MB zone anymore, yet we have hard time to get rid
> > > > of it and so we have that memory laying around unused all the time
> > > > and blocking one page flag bit. CMA falls into a similar category
> > > > AFAIU. I wouldn't be all that surprised if a future HW will not need CMA
> > > > allocations in few years, yet we will have to fight to get rid of it
> > > > like we do with ZONE_DMA. And not only that. We will also have to fight
> > > > finding page flags for other more general usecases in the meantime.
> > > 
> > > This maintenance problem is inherent. This problem exists even if we
> > > uses MIGRATETYPE approach. We cannot remove many hooks for CMA if a
> > > future HW will not need CMA allocation in few years. The only
> > > difference is that one takes single zone bit only for CMA user and the
> > > other approach takes many hooks that we need to take care about it all
> > > the time.
> > 
> > And I consider this a big difference. Because while hooks are not nice
> > they will affect CMA users (in a sense of bugs/performance etc.). While
> > an additional bit consumed will affect potential future and more generic
> > features.
> 
> Because these hooks are so tricky and are spread on many places,
> bugs about these hooks can be made by *non-CMA* user and they hurt
> *CMA* user. These hooks could also delay non-CMA user's development speed
> since there are many hooks about CMA and understanding how CMA is managed
> is rather difficult.

Than make those hooks easier to maintain. Seriously this is a
non-argument.

[...]

> > And all this can be isolated to CMA specific hooks with mostly minimum
> > impact to most users. I hear you saying that zone approach is more natural
> > and I would agree if we wouldn't have to care about the number of zones.
> 
> I attach a solution about one more bit in page flags although I don't
> agree with your opinion that additional bit is no-go approach. Just
> note that we have already used three bits for zone encoding in some
> configuration due to ZONE_DEVICE.

I am absolutely not happy about ZONE_DEVICE but there is _no_ other
viable solution right now. I know that people behind this are still
considering struct page over direct pfn usage but they are not in the
same situation as CMA which _can_ work without additional zone.

If you _really_ insist on using zone for CMA then reuse ZONE_MOVABLE.
I absolutely miss why do you push a specialized zone so hard.

[...]
> > No, but I haven't heard any single argument that those bugs are
> > impossible to fix with the current approach. They might be harder to fix
> > but if I can chose between harder for CMA and harder for other more
> > generic HW independent features I will go for the first one. And do not
> > take me wrong, I have nothing against CMA as such. It solves a real life
> > problem. I just believe it doesn't deserve to consume a new bit in page
> > flags because that is just too scarce resource.
> 
> As I mentioned above, I think that maintenance overhead due to CMA
> deserves to consume a new bit in page flags. It also provide us
> extendability to introduce more zones in the future.
> 
> Anyway, this value-judgement is subjective so I guess that we
> cannot agree with each other. To solve your concern,
> I make following solution. Please let me know your opinion about this.
> This patch can be applied on top of my ZONE_CMA series.

I don not think this makes situation any easier or more acceptable for
merging.

But I feel we are looping without much progress. So let me NAK this
until it is _proven_ that the current code is unfixable nor ZONE_MOVABLE
can be reused
-- 
Michal Hocko
SUSE Labs

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