linux-kernel - Re: [PATCH v5 1/5] mm,memory_hotplug: Allocate memmap from the added memory range

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Message-ID: <32bc6e31-0200-1e8c-895c-3f60ed072fc2@redhat.com>
Date:   Wed, 24 Mar 2021 14:13:31 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Oscar Salvador <osalvador@...e.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Pavel Tatashin <pasha.tatashin@...een.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 1/5] mm,memory_hotplug: Allocate memmap from the added
 memory range

On 24.03.21 13:37, Michal Hocko wrote:
> On Wed 24-03-21 13:23:47, David Hildenbrand wrote:
>> On 24.03.21 13:10, Michal Hocko wrote:
>>> On Wed 24-03-21 13:03:29, Michal Hocko wrote:
>>>> On Wed 24-03-21 11:12:59, Oscar Salvador wrote:
>>> [...]
>>>
>>> an additional remark
>>>
>>>>> - online_pages()->move_pfn_range_to_zone(): Accounts for node/zone's spanned pages
>>>>> - online_pages()->zone->present_pages += nr_pages;
>>>
>>> I am pretty sure you shouldn't account vmmemmap pages to the target zone
>>> in some cases - e.g. vmemmap cannot be part of the movable zone, can it?
>>> So this would be yet another special casing. This patch has got it wrong
>>> unless I have missed some special casing.
>>>
>>
>> It's a bit unfortunate that we have to discuss the very basic design
>> decisions again.
> 
> It would be great to have those basic design decisions layed out in the
> changelog.
> 
>> @Oscar, maybe you can share the links where we discussed all this and add
>> some of it to the patch description.
>>
>> I think what we have right here is good enough for an initial version, from
>> where on we can improve things without having to modify calling code.
> 
> I have to say I really dislike vmemmap proliferation into
> {on,off}lining. It just doesn't belong there from a layering POV. All
> this code should care about is to hand over pages to the allocator and
> make them visible.

Well, someone has to initialize the vmemmap of the vmemmap pages ( which 
is itself :) ), and as the vemmap does not span complete sections things 
can get very weird as we can only set whole sections online (there was 
more to that, I think it's buried in previous discussions).

> 
> Is that a sufficient concern to nack the whole thing? No, I do not think
> so. But I do not see any particular rush to have this work needs to be
> merged ASAP.

Sure, there is no need to rush (not that I suggested that).

-- 
Thanks,

David / dhildenb