Message-ID: <0e58dce3-d68f-f8cb-936e-c7ae49c1eeaa@redhat.com>
Date:   Wed, 24 Mar 2021 15:05:54 +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 14:40, Michal Hocko wrote:
> On Wed 24-03-21 14:13:31, David Hildenbrand wrote:
>> 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 :) ),
> 
> Yeah, and I would expect this to be done when the vmemmap space is
> reserved. This is at the hotadd time and we do not know the zone but
> that shouldn't really matter because their zone can be quite arbitrary
> kernel zone. As mentioned previously I do not think associating those
> with zone movable is a good idea as they are fundamentally not movable.

I don't think that's an issue. Just another special case to keep things 
simple. (and not completely fragment zones, mess with zone shrinking etc.)

> It is likely that the zone doesn't really matter for these pages anyway
> and the only think we do care about is that they are not poisoned and
> there is at least something but again it would be much better to have a
> single place where all those details are done (including accounting)
> rather than trying to wrap head around different pfns when onlining
> pages and grow potential and suble bugs there.

Exactly, as you said, the zone doesn't really matter - thus, this patch 
just handles it as simple as possible: keep them in the same zone as the 
hole memory block. No fragmented zones. no special casing. simple.

Details are actually pretty much all at a single place when 
onlining/offlining().

> 
>> 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).
> 
> Yes the section can only online as whole. This is an important "detail"
> and it would deserve some more clarification in the changelog as well.

Indeed.

> You have invested quite some energy into code consolidation and checks
> to make sure that hotplugged code doesn't have holes and this work bends
> those rules. vmemmap is effectivelly a hole in a memblock/section. I
> think we should re-evaluate some of those constrains rather than try to
> work them around somehow.
It's an offset in the beginning, so it's a special case. And the 
question is if there is a real benefit in handling it differently, for 
example, messing with online sections, messing with zones .. I am not 
convinced that the added complexity gives us a real benefit. But I shall 
be taught otherwise.


BTW: I once thought about having 
online_memory_block(block)/offline_memory_block(block) as separate 
functions instead of having pretty generic (error prone?) 
online_pages()/offline_pages(). Then, these details would just go in 
there and memory blocks + online/offline logic would simply be 
self-contained.

-- 
Thanks,

David / dhildenb

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