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: Wed, 24 Mar 2021 20:16:53 +0100 [thread overview]
Message-ID: <9591a0b8-c000-2f61-67a6-4402678fe50b@redhat.com> (raw)
In-Reply-To: <YFtjCMwYjx1BwEg0@dhcp22.suse.cz>
On 24.03.21 17:04, Michal Hocko wrote:
> On Wed 24-03-21 15:52:38, David Hildenbrand wrote:
>> On 24.03.21 15:42, Michal Hocko wrote:
>>> On Wed 24-03-21 13:03:29, Michal Hocko wrote:
>>>> On Wed 24-03-21 11:12:59, Oscar Salvador wrote:
>>> [...]
>>>>> I kind of understand to be reluctant to use vmemmap_pages terminology here, but
>>>>> unfortunately we need to know about it.
>>>>> We could rename nr_vmemmap_pages to offset_buddy_pages or something like that.
>>>>
>>>> I am not convinced. It seems you are justr trying to graft the new
>>>> functionality in. But I still believe that {on,off}lining shouldn't care
>>>> about where their vmemmaps come from at all. It should be a
>>>> responsibility of the code which reserves that space to compansate for
>>>> accounting. Otherwise we will end up with a hard to maintain code
>>>> because expectations would be spread at way too many places. Not to
>>>> mention different pfns that the code should care about.
>>>
>>> The below is a quick hack on top of this patch to illustrate my
>>> thinking. I have dug out all the vmemmap pieces out of the
>>> {on,off}lining and hooked all the accounting when the space is reserved.
>>> This just compiles without any deeper look so there are likely some
>>> minor problems but I haven't really encountered any major problems or
>>> hacks to introduce into the code. The separation seems to be possible.
>>> The diffstat also looks promising. Am I missing something fundamental in
>>> this?
>>>
>>
>> From a quick glimpse, this touches on two things discussed in the past:
>>
>> 1. If the underlying memory block is offline, all sections are offline. Zone
>> shrinking code will happily skip over the vmemmap pages and you can end up
>> with out-of-zone pages assigned to the zone. Can happen in corner cases.
>
> You are right. But do we really care? Those pages should be of no
> interest to anybody iterating through zones/nodes anyway.
Well, we were just discussing getting zone/node links + span right for
all pages (including for special reserved pages), because it already
resulted in BUGs. So I am not convinced that we *don't* have to care.
However, I agree that most code that cares about node/zone spans
shouldn't care - e.g., never call set_pfnblock_flags_mask() on such blocks.
But I guess there are corner cases where we would end up with
zone_is_empty() == true, not sure what that effect would be ... at least
the node cannot vanish as we disallow offlining it while we have a
memory block linked to it.
Another thing that comes to my mind is that our zone shrinking code
currently searches in PAGES_PER_SUBSECTION (2 MiB IIRC) increments. In
case our vmemmap pages would be less than that, we could accidentally
shrink the !vmemmap part too much, as we are mis-detecting the type for
a PAGES_PER_SUBSECTION block.
IIRC, this would apply for memory block sizes < 128 MiB. Not relevant on
x86 and arm64. Could be relevant for ppc64, if we'd ever want to support
memmap_on_memory there. Or if we'd ever reduce the section size on some
arch below 128 MiB. At least we would have to fence it somehow.
>
>> There is no way to know that the memmap of these pages was initialized and
>> is of value.
>>
>> 2. You heavily fragment zone layout although you might end up with
>> consecutive zones (e.g., online all hotplugged memory movable)
>
> What would be consequences?
IIRC, set_zone_contiguous() will leave zone->contiguous = false.
This, in turn, will force pageblock_pfn_to_page() via the slow path,
turning page isolation a bit slower.
Not a deal breaker, but obviously something where Oscar's original patch
can do better.
I yet have to think again about other issues (I remember most issues we
discussed back then were related to having the vmemmap only within the
same memory block). I think 2) might be tolerable, although unfortunate.
Regarding 1), we'll have to dive into more details.
--
Thanks,
David / dhildenb
next prev parent reply other threads:[~2021-03-24 19:18 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 [this message]
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
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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