From: David Hildenbrand <david@redhat.com>
To: Michal Hocko <mhocko@suse.com>
Cc: Oscar Salvador <osalvador@suse.de>,
Andrew Morton <akpm@linux-foundation.org>,
Anshuman Khandual <anshuman.khandual@arm.com>,
Vlastimil Babka <vbabka@suse.cz>,
Pavel Tatashin <pasha.tatashin@soleen.com>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v5 1/5] mm,memory_hotplug: Allocate memmap from the added memory range
Date: Thu, 25 Mar 2021 16:40:45 +0100 [thread overview]
Message-ID: <8e825d6a-bed5-d2ac-fc69-ae50ba787fe2@redhat.com> (raw)
In-Reply-To: <YFyt3UfoPkt7BbDZ@dhcp22.suse.cz>
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
next prev parent reply other threads:[~2021-03-25 15:41 UTC|newest]
Thread overview: 63+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-19 9:26 [PATCH v5 0/5] Allocate memmap from hotadded memory (per device) Oscar Salvador
2021-03-19 9:26 ` [PATCH v5 1/5] mm,memory_hotplug: Allocate memmap from the added memory range Oscar Salvador
2021-03-19 10:20 ` David Hildenbrand
2021-03-19 10:31 ` Oscar Salvador
2021-03-19 12:04 ` David Hildenbrand
2021-03-23 10:11 ` Michal Hocko
2021-03-24 10:12 ` Oscar Salvador
2021-03-24 12:03 ` Michal Hocko
2021-03-24 12:10 ` Michal Hocko
2021-03-24 12:23 ` David Hildenbrand
2021-03-24 12:37 ` Michal Hocko
2021-03-24 13:13 ` David Hildenbrand
2021-03-24 13:40 ` Michal Hocko
2021-03-24 14:05 ` David Hildenbrand
2021-03-24 13:27 ` Oscar Salvador
2021-03-24 14:42 ` Michal Hocko
2021-03-24 14:52 ` David Hildenbrand
2021-03-24 16:04 ` Michal Hocko
2021-03-24 19:16 ` David Hildenbrand
2021-03-25 8:07 ` Oscar Salvador
2021-03-25 9:17 ` Michal Hocko
2021-03-25 10:55 ` Oscar Salvador
2021-03-25 11:08 ` David Hildenbrand
2021-03-25 11:23 ` Oscar Salvador
2021-03-25 12:35 ` Michal Hocko
2021-03-25 12:40 ` David Hildenbrand
2021-03-25 14:08 ` Michal Hocko
2021-03-25 14:09 ` David Hildenbrand
2021-03-25 14:34 ` Michal Hocko
2021-03-25 14:46 ` David Hildenbrand
2021-03-25 15:12 ` Michal Hocko
2021-03-25 15:19 ` David Hildenbrand
2021-03-25 15:35 ` Michal Hocko
2021-03-25 15:40 ` David Hildenbrand [this message]
2021-03-25 16:07 ` Michal Hocko
2021-03-25 16:20 ` David Hildenbrand
2021-03-25 16:36 ` Michal Hocko
2021-03-25 16:47 ` Michal Hocko
2021-03-25 16:55 ` David Hildenbrand
2021-03-25 22:06 ` Oscar Salvador
2021-03-26 8:35 ` Michal Hocko
2021-03-26 8:52 ` David Hildenbrand
2021-03-26 8:57 ` Oscar Salvador
2021-03-26 12:15 ` Oscar Salvador
2021-03-26 13:36 ` David Hildenbrand
2021-03-26 14:38 ` Michal Hocko
2021-03-26 14:53 ` David Hildenbrand
2021-03-26 15:31 ` Michal Hocko
2021-03-26 16:03 ` David Hildenbrand
2021-03-26 8:55 ` Oscar Salvador
2021-03-26 9:11 ` Michal Hocko
2021-03-25 18:08 ` David Hildenbrand
2021-03-25 12:26 ` Michal Hocko
2021-03-25 14:02 ` Oscar Salvador
2021-03-25 14:40 ` Michal Hocko
2021-03-19 9:26 ` [PATCH v5 2/5] acpi,memhotplug: Enable MHP_MEMMAP_ON_MEMORY when supported Oscar Salvador
2021-03-23 10:40 ` Michal Hocko
2021-03-19 9:26 ` [PATCH v5 3/5] mm,memory_hotplug: Add kernel boot option to enable memmap_on_memory Oscar Salvador
2021-03-23 10:47 ` Michal Hocko
2021-03-24 8:45 ` Oscar Salvador
2021-03-24 9:02 ` Michal Hocko
2021-03-19 9:26 ` [PATCH v5 4/5] x86/Kconfig: Introduce ARCH_MHP_MEMMAP_ON_MEMORY_ENABLE Oscar Salvador
2021-03-19 9:26 ` [PATCH v5 5/5] arm64/Kconfig: " Oscar Salvador
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