Date:   Thu, 25 Mar 2021 12:08:43 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Oscar Salvador <osalvador@...e.de>, Michal Hocko <mhocko@...e.com>
Cc:     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 25.03.21 11:55, Oscar Salvador wrote:
> On Thu, Mar 25, 2021 at 10:17:33AM +0100, Michal Hocko wrote:
>> Why do you think it is wrong to initialize/account pages when they are
>> used? Keep in mind that offline pages are not used until they are
>> onlined. But vmemmap pages are used since the vmemmap is established
>> which happens in the hotadd stage.
> 
> Yes, that is true.
> vmemmap pages are used right when we populate the vmemmap space.
> 

Note: I once herd of a corner-case use case where people offline memory 
blocks to then use the "free" memory via /dev/mem for other purposes 
("large physical memory"). Not that I encourage such use cases, but they 
would be fundamentally broken if the vmemmap ends up on offline memory 
and is supposed to keep its state ...

>>> plus the fact that I dislike to place those pages in
>>> ZONE_NORMAL, although they are not movable.
>>> But I think the vmemmap pages should lay within the same zone the pages
>>> they describe, doing so simplifies things, and I do not see any outright
>>> downside.
>>
>> Well, both ways likely have its pros and cons. Nevertheless, if the
>> vmemmap storage is independent (which is the case for normal hotplug)
>> then the state is consistent over hotadd, {online, offline} N times,
>> hotremove cycles.  Which is conceptually reasonable as vmemmap doesn't
>> go away on each offline.
>>
>> If you are going to bind accounting to the online/offline stages then
>> the accounting changes each time you go through the cycle and depending
>> on the onlining type it would travel among zones. I find it quite
>> confusing as the storage for vmemmap hasn't changed any of its
>> properties.
> 
> That is a good point I guess.
> vmemmap pages do not really go away until the memory is unplugged.
> 
> But I see some questions to raise:
> 
> - As I said, I really dislike it tiding vmemmap memory to ZONE_NORMAL
>    unconditionally and this might result in the problems David mentioned.
>    I remember David and I discussed such problems but the problems with
>    zones not being contiguos have also been discussed in the past and
>    IIRC, we reached the conclusion that a maximal effort should be made
>    to keep them that way, otherwise other things suffer e.g: compaction
>    code.
>    So if we really want to move the initialization/account to the
>    hot-add/hot-remove stage, I would really like to be able to set the
>    proper zone in there (that is, the same zone where the memory will lay).

Determining the zone when hot-adding does not make too much sense: you 
don't know what user space might end up deciding (online_kernel, 
online_movable...).

> 
> - When moving the initialization/accounting to hot-add/hot-remove,
>    the section containing the vmemmap pages will remain offline.
>    It might get onlined once the pages get online in online_pages(),
>    or not if vmemmap pages span a whole section.
>    I remember (but maybe David rmemeber better) that that was a problem
>    wrt. pfn_to_online_page() and hybernation/kdump.
>    So, if that is really a problem, we would have to care of ot setting
>    the section to the right state.

Good memory. Indeed, hibernation/kdump won't save the state of the 
vmemmap, because the memory is marked as offline and, thus, logically 
without any valuable content.

> 
> - AFAICS, doing all the above brings us to former times were some
>    initialization/accounting was done in a previous stage, and I remember
>    it was pushed hard to move those in online/offline_pages().
>    Are we ok with that?
>    As I said, we might have to set the right zone in hot-add stage, as
>    otherwise problems might come up.
>    Being that case, would not that also be conflating different concepts
>    at a wrong phases?
> 

I expressed my opinion already, no need to repeat. Sub-section online 
maps would make it cleaner, but I am still not convinced we want/need that.

> Do not take me wrong, I quite like Michal's idea, and from a
> conceptually point of view I guess it is the right thing to do.
> But when evualating risks/difficulty, I am not really sure.
> 
> If we can pull that off while setting the right zone (and must be seen
> what about the section state), and the outcome is not ugly, I am all for
> it.
> Also a middel-ground might be something like I previously mentioned(having
> a helper in memory_block_action() to do the right thing, so
> offline/online_pages() do not get pouled.

As I said, having soemthing like 
memory_block_online()/memory_block_offline() could be one way to tackle 
it. We only support onlining/offlining of memory blocks and I ripped out 
all code that was abusing online_pages/offline_pages ...

So have memory_block_online() call online_pages() and do the accounting 
of the vmemmap, with a big fat comment that sections are actually set 
online/offline in online_pages/offline_pages(). Could be a simple 
cleanup on top of this series ...


-- 
Thanks,

David / dhildenb

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