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Subject: Re: [PATCH v1] powerpc/pseries: CMM: Drop page array
From:   David Hildenbrand <david@redhat.com>
To:     linux-kernel@vger.kernel.org
Cc:     linux-mm@kvack.org, linuxppc-dev@lists.ozlabs.org,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Arun KS <arunks@codeaurora.org>,
        Pavel Tatashin <pasha.tatashin@soleen.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Michal Hocko <mhocko@kernel.org>
References: <20190910163932.13160-1-david@redhat.com>
 <a2c2f516-c37c-71f5-8f35-c357e8754b17@redhat.com>
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Date:   Fri, 27 Sep 2019 13:19:49 +0200
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On 25.09.19 09:37, David Hildenbrand wrote:
> On 10.09.19 18:39, David Hildenbrand wrote:
>> We can simply store the pages in a list (page->lru), no need for a
>> separate data structure (+ complicated handling). This is how most
>> other balloon drivers store allocated pages without additional tracking
>> data.
>>
>> For the notifiers, use page_to_pfn() to check if a page is in the
>> applicable range. plpar_page_set_loaned()/plpar_page_set_active() were
>> called with __pa(page_address()) for now, I assume we can simply switch
>> to page_to_phys() here. The pfn_to_kaddr() handling is now mostly gone.
>>
>> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
>> Cc: Paul Mackerras <paulus@samba.org>
>> Cc: Michael Ellerman <mpe@ellerman.id.au>
>> Cc: Arun KS <arunks@codeaurora.org>
>> Cc: Pavel Tatashin <pasha.tatashin@soleen.com>
>> Cc: Thomas Gleixner <tglx@linutronix.de>
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> Cc: Vlastimil Babka <vbabka@suse.cz>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>> ---
>>
>> Only compile-tested. I hope the page_to_phys() thingy is correct and I
>> didn't mess up something else / ignoring something important why the array
>> is needed.
>>
>> I stumbled over this while looking at how the memory isolation notifier is
>> used - and wondered why the additional array is necessary. Also, I think
>> by switching to the generic balloon compaction mechanism, we could get
>> rid of the memory hotplug notifier and the memory isolation notifier in
>> this code, as the migration capability of the inflated pages is the real
>> requirement:
>> 	commit 14b8a76b9d53346f2871bf419da2aaf219940c50
>> 	Author: Robert Jennings <rcj@linux.vnet.ibm.com>
>> 	Date:   Thu Dec 17 14:44:52 2009 +0000
>> 	
>> 	    powerpc: Make the CMM memory hotplug aware
>> 	
>> 	    The Collaborative Memory Manager (CMM) module allocates individual pages
>> 	    over time that are not migratable.  On a long running system this can
>> 	    severely impact the ability to find enough pages to support a hotplug
>> 	    memory remove operation.
>> 	[...]
>>
>> Thoughts?
> 
> Ping, is still feature still used at all?
> 
> If nobody can test, any advise on which HW I need and how to trigger it?
> 

So ... if CMM is no longer alive I propose ripping it out completely.
Does anybody know if this feature is still getting used? Getting rid of
the memory isolation notifier sounds desirable - either by scrapping CMM
or by properly wiring up balloon compaction.

-- 

Thanks,

David / dhildenb