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Subject: Re: [PATCH v1 0/9] s390: implement and optimize vmemmap_free()
To:     Heiko Carstens <heiko.carstens@de.ibm.com>
Cc:     linux-kernel@vger.kernel.org, linux-s390@vger.kernel.org,
        linux-mm@kvack.org, Christian Borntraeger <borntraeger@de.ibm.com>,
        Gerald Schaefer <gerald.schaefer@de.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>
References: <20200703133917.39045-1-david@redhat.com>
 <20200707120849.GB12303@osiris>
From:   David Hildenbrand <david@redhat.com>
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Message-ID: <f4a87c47-4987-e3f8-8c06-ff6dd60f6a39@redhat.com>
Date:   Tue, 7 Jul 2020 14:13:37 +0200
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On 07.07.20 14:08, Heiko Carstens wrote:
> On Fri, Jul 03, 2020 at 03:39:08PM +0200, David Hildenbrand wrote:
>> This series is based on the latest s390/features branch [1]. It implements
>> vmemmap_free(), consolidating it with vmem_add_range(), and optimizes it by
>> - Freeing empty page tables (now also done for idendity mapping).
>> - Handling cases where the vmemmap of a section does not fill huge pages
>>   completely.
>>
>> vmemmap_free() is currently never used, unless adiing standby memory fails
>> (unlikely). This is relevant for virtio-mem, which adds/removes memory
>> in memory block/section granularity (always removes memory in the same
>> granularity it added it).
>>
>> I gave this a proper test with my virtio-mem prototype (which I will share
>> once the basic QEMU implementation is upstream), both with 56 byte memmap
>> per page and 64 byte memmap per page, with and without huge page support.
>> In both cases, removing memory (routed through arch_remove_memory()) will
>> result in
>> - all populated vmemmap pages to get removed/freed
>> - all applicable page tables for the vmemmap getting removed/freed
>> - all applicable page tables for the idendity mapping getting removed/freed
>> Unfortunately, I don't have access to bigger and z/VM (esp. dcss)
>> environments.
>>
>> This is the basis for real memory hotunplug support for s390x and should
>> complete my journey to s390x vmem/vmemmap code for now :)
>>
>> What needs double-checking is tlb flushing. AFAIKS, as there are no valid
>> accesses, doing a single range flush at the end is sufficient, both when
>> removing vmemmap pages and the idendity mapping.
>>
>> Along, some minor cleanups.
> 
> Hmm.. I really would like to see if there would be only a single page
> table walker left in vmem.c, which handles both adding and removing
> things.
> Now we end up with two different page table walk implementations
> within the same file. However not sure if it is worth the effort to
> unify them though.

I tried to unify vmemmap_populate() and vmem_add_range() already and
didn't like the end result ... so, unifying these along with the removal
part won't be any better - most probably. Open for suggestions :)

(at least arm64 and x86-64 handle it similarly)

-- 
Thanks,

David / dhildenb