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

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Message-ID: <40fac999-2d28-9205-23f0-516fa9342bbe@redhat.com>
Date:   Thu, 25 Mar 2021 16:19:36 +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 16:12, 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.

-- 
Thanks,

David / dhildenb