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

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Message-ID: <8e825d6a-bed5-d2ac-fc69-ae50ba787fe2@redhat.com>
Date:   Thu, 25 Mar 2021 16:40:45 +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 25.03.21 16:35, Michal Hocko wrote:
> On Thu 25-03-21 16:19:36, David Hildenbrand wrote:
>> On 25.03.21 16:12, Michal Hocko wrote:
>>> On Thu 25-03-21 15:46:22, David Hildenbrand wrote:
>>>> On 25.03.21 15:34, Michal Hocko wrote:
>>>>> On Thu 25-03-21 15:09:35, David Hildenbrand wrote:
>>>>>> On 25.03.21 15:08, Michal Hocko wrote:
>>>>>>> On Thu 25-03-21 13:40:45, David Hildenbrand wrote:
>>>>>>>> On 25.03.21 13:35, Michal Hocko wrote:
>>>>>>>>> On Thu 25-03-21 12:08:43, David Hildenbrand wrote:
>>>>>>>>>> On 25.03.21 11:55, Oscar Salvador wrote:
>>>>>>> [...]
>>>>>>>>>>> - 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.
>>>>>
>>>>> ^^^^ THIS
>>>>>
>>>>>>>>>
>>>>>>>>> Could you point me to the respective hibernation code please? I always
>>>>>>>>> get lost in that area. Anyway, we do have the same problem even if the
>>>>>>>>> whole accounting is handled during {on,off}lining, no?
>>>>>>>>
>>>>>>>> kernel/power/snapshot.c:saveable_page().
>>>>>>>
>>>>>>> Thanks! So this is as I've suspected. The very same problem is present
>>>>>>> if the memory block is marked offline. So we need a solution here
>>>>>>> anyway. One way to go would be to consider these vmemmap pages always
>>>>>>> online. pfn_to_online_page would have to special case them but we would
>>>>>>> need to identify them first. I used to have PageVmemmap or something
>>>>>>> like that in my early attempt to do this.
>>>>>>>
>>>>>>> That being said this is not an argument for one or the other aproach.
>>>>>>> Both need fixing.
>>>>>>
>>>>>> Can you elaborate? What is the issue there? What needs fixing?
>>>>>
>>>>> offline section containing vmemmap will be lost during hibernation cycle
>>>>> IIU the above correctly.
>>>>>
>>>>
>>>> Can tell me how that is a problem with Oscars current patch? I only see this
>>>> being a problem with what you propose - most probably I am missing something
>>>> important here.
>>>>
>>>> Offline memory sections don't have a valid memmap (assumption: garbage). On
>>>> hibernation, the whole offline memory block won't be saved, including the
>>>> vmemmap content that resides on the block. This includes the vmemmap of the
>>>> vmemmap pages, which is itself.
>>>>
>>>> When restoring, the whole memory block will contain garbage, including the
>>>> whole vmemmap - which is marked to be offline and to contain garbage.
>>>
>>> Hmm, so I might be misunderstanding the restoring part. But doesn't that
>>> mean that the whole section/memory block won't get restored because it
>>> is offline and therefore the vmemmap would be pointing to nowhere?
>>
>> AFAIU, only the content of the memory block won't be restored - whatever
>> memory content existed before the restore operation is kept.
>>
>> The structures that define how the vmemmap should look like - the memory
>> sections and the page tables used for describing the vmemmap should properly
>> get saved+restored, as these are located on online memory.
>>
>> So the vmemmap layout should look after restoring just like before saving.
> 
> OK, makes sense. Thanks for the clarification.
> 
> So there is indeed a difference. One way around that would be to mark
> vmemmap pages (e.g. PageReserved && magic value stored somewhere in the
> struct page - resembling bootmem vmemmaps) or mark section fully backing
> vmemmaps as online (ugly).

I'm sorry Michal, but then we are hacking around the online section size 
limitation just in another (IMHO more ugly) way, then what Oscar and I 
came up with when discussing this in the past.

Your first approach would require us to look at potential garbage 
(pfn_to_online_page() == NULL) and filter out what might still be useful.

The second approach exposes garbage to the rest of the system as 
initialized memmap.


I honestly cannot say that I prefer either over what we have here.

-- 
Thanks,

David / dhildenb