From: Michal Hocko <mhocko@suse.com>
To: Oscar Salvador <osalvador@suse.de>
Cc: David Hildenbrand <david@redhat.com>,
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 13:26:34 +0100 [thread overview]
Message-ID: <YFyBeulR04Oc7eu2@dhcp22.suse.cz> (raw)
In-Reply-To: <YFxsBRORtgqUF/FZ@localhost.localdomain>
On Thu 25-03-21 11:55:01, 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.
>
> > > plus the fact that I dislike to place those pages in
> > > ZONE_NORMAL, although they are not movable.
> > > But I think the vmemmap pages should lay within the same zone the pages
> > > they describe, doing so simplifies things, and I do not see any outright
> > > downside.
> >
> > Well, both ways likely have its pros and cons. Nevertheless, if the
> > vmemmap storage is independent (which is the case for normal hotplug)
> > then the state is consistent over hotadd, {online, offline} N times,
> > hotremove cycles. Which is conceptually reasonable as vmemmap doesn't
> > go away on each offline.
> >
> > If you are going to bind accounting to the online/offline stages then
> > the accounting changes each time you go through the cycle and depending
> > on the onlining type it would travel among zones. I find it quite
> > confusing as the storage for vmemmap hasn't changed any of its
> > properties.
>
> That is a good point I guess.
> vmemmap pages do not really go away until the memory is unplugged.
>
> But I see some questions to raise:
>
> - As I said, I really dislike it tiding vmemmap memory to ZONE_NORMAL
> unconditionally and this might result in the problems David mentioned.
> I remember David and I discussed such problems but the problems with
> zones not being contiguos have also been discussed in the past and
> IIRC, we reached the conclusion that a maximal effort should be made
> to keep them that way, otherwise other things suffer e.g: compaction
> code.
Yeah, David has raised the contiguous flag for zone already. And to be
completely honest I fail to see why we should shape a design based on an
optimization. If anything we can teach set_zone_contiguous to simply
ignore zone affiliation of vmemmap pages. I would be really curious if
that would pose any harm to the compaction code as they are reserved and
compaction should simply skip them.
> So if we really want to move the initialization/account to the
> hot-add/hot-remove stage, I would really like to be able to set the
> proper zone in there (that is, the same zone where the memory will lay).
THere is nothing like a proper zone.
> - When moving the initialization/accounting to hot-add/hot-remove,
> the section containing the vmemmap pages will remain offline.
Yes this sucks! I do not have a good answer for that as the
online/offline granularity seems rather coarse on that.
> 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.
>
> - 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().
Not sure what you are referring to but if you have prior to f1dd2cd13c4b
("mm, memory_hotplug: do not associate hotadded memory to zones until
online") then this was entirely a different story. Users do care where
they memory goes because that depends on the usecase but do they care
about vmemmap?
--
Michal Hocko
SUSE Labs
next prev parent reply other threads:[~2021-03-25 12:27 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
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 [this message]
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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