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

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Date:   Wed, 24 Mar 2021 20:16:53 +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 17:04, Michal Hocko wrote:
> On Wed 24-03-21 15:52:38, David Hildenbrand wrote:
>> On 24.03.21 15:42, 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:
>>> [...]
>>>>> I kind of understand to be reluctant to use vmemmap_pages terminology here, but
>>>>> unfortunately we need to know about it.
>>>>> We could rename nr_vmemmap_pages to offset_buddy_pages or something like that.
>>>>
>>>> I am not convinced. It seems you are justr trying to graft the new
>>>> functionality in. But I still believe that {on,off}lining shouldn't care
>>>> about where their vmemmaps come from at all. It should be a
>>>> responsibility of the code which reserves that space to compansate for
>>>> accounting. Otherwise we will end up with a hard to maintain code
>>>> because expectations would be spread at way too many places. Not to
>>>> mention different pfns that the code should care about.
>>>
>>> The below is a quick hack on top of this patch to illustrate my
>>> thinking. I have dug out all the vmemmap pieces out of the
>>> {on,off}lining and hooked all the accounting when the space is reserved.
>>> This just compiles without any deeper look so there are likely some
>>> minor problems but I haven't really encountered any major problems or
>>> hacks to introduce into the code. The separation seems to be possible.
>>> The diffstat also looks promising. Am I missing something fundamental in
>>> this?
>>>
>>
>>  From a quick glimpse, this touches on two things discussed in the past:
>>
>> 1. If the underlying memory block is offline, all sections are offline. Zone
>> shrinking code will happily skip over the vmemmap pages and you can end up
>> with out-of-zone pages assigned to the zone. Can happen in corner cases.
> 
> You are right. But do we really care? Those pages should be of no
> interest to anybody iterating through zones/nodes anyway.

Well, we were just discussing getting zone/node links + span right for 
all pages (including for special reserved pages), because it already 
resulted in BUGs. So I am not convinced that we *don't* have to care.

However, I agree that most code that cares about node/zone spans 
shouldn't care - e.g., never call set_pfnblock_flags_mask() on such blocks.

But I guess there are corner cases where we would end up with 
zone_is_empty() == true, not sure what that effect would be ... at least 
the node cannot vanish as we disallow offlining it while we have a 
memory block linked to it.

Another thing that comes to my mind is that our zone shrinking code 
currently searches in PAGES_PER_SUBSECTION (2 MiB IIRC) increments. In 
case our vmemmap pages would be less than that, we could accidentally 
shrink the !vmemmap part too much, as we are mis-detecting the type for 
a PAGES_PER_SUBSECTION block.

IIRC, this would apply for memory block sizes < 128 MiB. Not relevant on 
x86 and arm64. Could be relevant for ppc64, if we'd ever want to support 
memmap_on_memory there. Or if we'd ever reduce the section size on some 
arch below 128 MiB. At least we would have to fence it somehow.

> 
>> There is no way to know that the memmap of these pages was initialized and
>> is of value.
>>
>> 2. You heavily fragment zone layout although you might end up with
>> consecutive zones (e.g., online all hotplugged memory movable)
> 
> What would be consequences?

IIRC, set_zone_contiguous() will leave zone->contiguous = false.

This, in turn, will force pageblock_pfn_to_page() via the slow path, 
turning page isolation a bit slower.

Not a deal breaker, but obviously something where Oscar's original patch 
can do better.

I yet have to think again about other issues (I remember most issues we 
discussed back then were related to having the vmemmap only within the 
same memory block). I think 2) might be tolerable, although unfortunate. 
Regarding 1), we'll have to dive into more details.

-- 
Thanks,

David / dhildenb