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

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Message-ID: <YFy8ARml4R7/snVs@dhcp22.suse.cz>
Date:   Thu, 25 Mar 2021 17:36:17 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     David Hildenbrand <david@...hat.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 Thu 25-03-21 17:20:23, David Hildenbrand wrote:
> On 25.03.21 17:07, Michal Hocko wrote:
> > On Thu 25-03-21 16:35:58, Michal Hocko wrote:
> > [...]
> > > 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 am not yet ready to give up on this. Here is a quick stab at the
> > pfn_to_online_page approach. It is not great but it is not really
> > terrible either. I think we can do better and skip
> 
> We both seem to have a different taste, to phrase it in a nice way :) ; but
> well, you seem to have set your mind (just like I seem to have set mine when
> trying to find a nice and somewhat-clean way to handle this when discussing
> it in the past).

I definitely do not want to fight for a certain solution just for the
sake of it. I really dislike how the lifetime of the reserved space and
its accounting are completely detached. But hey, I do understand that
a worse solution from the design perspective can be better due to
practical reasons or constrains.

I haven't seen the hibernation problem before and I do recognize it is
a nasty one. If all it takes is to make pfn_to_online_page work (and my
previous attempt is incorrect because it should consult block rather
than section pfn range) and there are no other downsides then I would
still prefer to go with my proposal.  If there are still other things to
plug then, well, practicality is going to win.

So before I give up on the "proper" design card, are there more
subtleties to watch for? You have certainly given this much more thought
than I have.
-- 
Michal Hocko
SUSE Labs