Re: [PATCH v7 0/7] Introduce ZONE_CMA

[Michal Hocko <mhocko@kernel.org>]

On Mon 17-04-17 11:02:12, Joonsoo Kim wrote:
> On Thu, Apr 13, 2017 at 01:56:15PM +0200, Michal Hocko wrote:
> > On Wed 12-04-17 10:35:06, Joonsoo Kim wrote:
[...]
> > > ZONE_CMA is conceptually the same with ZONE_MOVABLE. There is a software
> > > constraint to guarantee the success of future allocation request from
> > > the device. If the device requests the specific range of the memory in CMA
> > > area at the runtime, page that allocated by MM will be migrated to
> > > the other page and it will be returned to the device. To guarantee it,
> > > ZONE_CMA only takes the allocation request with GFP_MOVABLE.
> > 
> > The immediate follow up question is. Why cannot we reuse ZONE_MOVABLE
> > for that purpose?
> 
> I can make CMA reuses the ZONE_MOVABLE but I don't want it. Reasons
> are that
> 
> 1. If ZONE_MOVABLE has two different types of memory, hotpluggable and
> CMA, it may need special handling for each type. This would lead to a new
> migratetype again (to distinguish them) and easy to be error-prone. I
> don't want that case.

Hmm, I see your motivation. I believe that we could find a way
around this. Anyway, movable zones are quite special and configuring
overlapping CMA and hotplug movable regions could be refused. So I am
not even sure this is a real problem in practice.

> 2. CMA users want to see usage stat separately since CMA often causes
> the problems and separate stat would helps to debug it.

That could be solved by a per-zone/node counter.

Anyway, these reasons should be mentioned as well. Adding a new zone is
not for free. For most common configurations where we have ZONE_DMA,
ZONE_DMA32, ZONE_NORMAL and ZONE_MOVABLE all the 3 bits are already
consumed so a new zone will need a new one AFAICS.

[...]
> > > Other things are completely the same with other zones. For MM POV, there is
> > > no difference in allocation process except that it only takes
> > > GFP_MOVABLE request. In reclaim, pages that are allocated by MM will
> > > be reclaimed by the same policy of the MM. So, no difference.
> > 
> > OK, so essentially this is yet another "highmem" zone. We already know
> > that only GFP_MOVABLE are allowed to fallback to ZONE_CMA but do CMA
> > allocations fallback to other zones and punch new holes? In which zone
> > order?
> 
> Hmm... I don't understand your question. Could you elaborate it more?

Well, my question was about the zone fallback chain. MOVABLE allocation
can fallback to lower zones and also to the ZONE_CMA with your patch. If
there is a CMA allocation it doesn't fall back to any other zone - in
other words no new holes are punched to other zones. Is this correct?

> > > This 'no difference' is a strong point of this approach. ZONE_CMA is
> > > naturally handled by MM subsystem unlike as before (special handling is
> > > required for MIGRATE_CMA).
> > > 
> > > 3. Controversial Point
> > > 
> > > Major concern from Mel is that zone concept is abused. ZONE is originally
> > > introduced to solve some issues due to H/W addressing limitation.
> > 
> > Yes, very much agreed on that. You basically want to punch holes into
> > other zones to guarantee an allocation progress. Marking those wholes
> > with special migrate type sounds quite natural but I will have to study
> > the current code some more to see whether issues you mention are
> > inherently unfixable. This might very well turn out to be the case.
> 
> At a glance, special migratetype sound natural. I also did. However,
> it's not natural in implementation POV. Zone consists of the same type
> of memory (by definition ?) and MM subsystem is implemented with that
> assumption. If difference type of memory shares the same zone, it easily
> causes the problem and CMA problems are the such case.

But this is not any different from the highmem vs. lowmem problems we
already have, no? I have looked at your example in the cover where you
mention utilization and the reclaim problems. With the node reclaim we
will have pages from all zones on the same LRU(s). isolate_lru_pages
will skip those from ZONE_CMA because their zone_idx is higher than
gfp_idx(GFP_KERNEL). The same could be achieved by an explicit check for
the pageblock migrate type. So the zone doesn't really help much. Or is
there some aspect that I am missing?

Another worry I would have with the zone approach is that there is a
risk to reintroduce issues we used to have with small zones in the
past. Just consider that the CMA will get depleted by CMA users almost
completely. Now that zone will not get balanced with only few pages.
wakeup_kswapd/pgdat_balanced already has measures to prevent from wake
ups but I cannot say I would be sure everything will work smoothly.

I have glanced through the cumulative diff and to be honest I am not
really sure the result is a great simplification in the end. There is
still quite a lot of special casing. It is true that the page allocator
path is cleaned up and some CMA specific checks are moved away. This is
definitely good to see but I am not convinced that the new zone is
really justified. Only very little from the zone infrastructure is used
in the end AFAICS. Is there any specific usecase which cannot be solved
with the pageblock while it could be with the zone approach? That would
be a strong argument to chose one over another.

Please do _not_ take this as a NAK from me. At least not at this time. I
am still trying to understand all the consequences but my intuition
tells me that building on top of highmem like approach will turn out to
be problematic in future (as we have already seen with the highmem and
movable zones) so this needs a very prudent consideration.
-- 
Michal Hocko
SUSE Labs

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