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 13:40:45 +0100 [thread overview]
Message-ID: <31110e58-c99a-8dee-6f6e-98f456b77759@redhat.com> (raw)
In-Reply-To: <YFyDhKPqS15HdO0Y@dhcp22.suse.cz>
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:
>>> On Thu, Mar 25, 2021 at 10:17:33AM +0100, Michal Hocko wrote:
>>>> Why do you think it is wrong to initialize/account pages when they are
>>>> used? Keep in mind that offline pages are not used until they are
>>>> onlined. But vmemmap pages are used since the vmemmap is established
>>>> which happens in the hotadd stage.
>>>
>>> Yes, that is true.
>>> vmemmap pages are used right when we populate the vmemmap space.
>>>
>>
>> Note: I once herd of a corner-case use case where people offline memory
>> blocks to then use the "free" memory via /dev/mem for other purposes ("large
>> physical memory"). Not that I encourage such use cases, but they would be
>> fundamentally broken if the vmemmap ends up on offline memory and is
>> supposed to keep its state ...
>
> I am not aware of such a use case, it surely sounds, ehm creative, but
> nothing really new. But such a usecase sounds quite incompatible with
> this feature whether we account vmemmap at hotadd or hotremove because
> they would need to understand that part of the memory they have hotadded
> is not useable.
I think they can use it just fine via /dev/mem, which explicitly avoids
any kind of "struct page" references IIRC. They would be overwriting the
vmemmap, but that part scan happily read/write until onlining, where the
vmemmap would get reinitialized and set online - from which point on
pfn_to_online_page() would succeed also on the vmemmap itself.
>
> [...]
>>> - 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().
>
> I am not really worried about kdump though. As the memory is offline
> then losing itse vmemmap is a mere annoyance.
Yep, kdump was relevant in our previous discussions when we were talking
about memory blocks having their altmap located on other memory blocks.
>
>
>>> - AFAICS, doing all the above brings us to former times were some
>>> initialization/accounting was done in a previous stage, and I remember
>>> it was pushed hard to move those in online/offline_pages().
>>> Are we ok with that?
>>> As I said, we might have to set the right zone in hot-add stage, as
>>> otherwise problems might come up.
>>> Being that case, would not that also be conflating different concepts
>>> at a wrong phases?
>>>
>>
>> I expressed my opinion already, no need to repeat. Sub-section online maps
>> would make it cleaner, but I am still not convinced we want/need that.
>
> Nah, subsections are more tricky than necessary and if we can live
> without them and have it just as pmem weirdness is more than enough ;)
Yes, absolutely :)
--
Thanks,
David / dhildenb
next prev parent reply other threads:[~2021-03-25 12:40 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 [this message]
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
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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