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

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Message-ID: <062bc5d7-a83c-1c1a-7b77-9f043643f4fa@redhat.com>
Date:   Thu, 25 Mar 2021 15:09:35 +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 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.
>>>
>>> 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?

-- 
Thanks,

David / dhildenb