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

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Date:   Wed, 24 Mar 2021 13:37:35 +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 Wed 24-03-21 13:23:47, David Hildenbrand wrote:
> On 24.03.21 13:10, 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:
> > [...]
> > 
> > an additional remark
> > 
> > > > - online_pages()->move_pfn_range_to_zone(): Accounts for node/zone's spanned pages
> > > > - online_pages()->zone->present_pages += nr_pages;
> > 
> > I am pretty sure you shouldn't account vmmemmap pages to the target zone
> > in some cases - e.g. vmemmap cannot be part of the movable zone, can it?
> > So this would be yet another special casing. This patch has got it wrong
> > unless I have missed some special casing.
> > 
> 
> It's a bit unfortunate that we have to discuss the very basic design
> decisions again.

It would be great to have those basic design decisions layed out in the
changelog.

> @Oscar, maybe you can share the links where we discussed all this and add
> some of it to the patch description.
> 
> I think what we have right here is good enough for an initial version, from
> where on we can improve things without having to modify calling code.

I have to say I really dislike vmemmap proliferation into
{on,off}lining. It just doesn't belong there from a layering POV. All
this code should care about is to hand over pages to the allocator and
make them visible.

Is that a sufficient concern to nack the whole thing? No, I do not think
so. But I do not see any particular rush to have this work needs to be
merged ASAP.

-- 
Michal Hocko
SUSE Labs