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

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Date:   Thu, 25 Mar 2021 15:34:28 +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 15:09:35, David Hildenbrand wrote:
> On 25.03.21 15:08, Michal Hocko wrote:
> > On Thu 25-03-21 13:40:45, David Hildenbrand wrote:
> > > 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:
> > [...]
> > > > > > - 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.

^^^^ THIS

> > > > 
> > > > 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().
> > 
> > Thanks! So this is as I've suspected. The very same problem is present
> > if the memory block is marked offline. So we need a solution here
> > anyway. One way to go would be to consider these vmemmap pages always
> > online. pfn_to_online_page would have to special case them but we would
> > need to identify them first. I used to have PageVmemmap or something
> > like that in my early attempt to do this.
> > 
> > That being said this is not an argument for one or the other aproach.
> > Both need fixing.
> 
> Can you elaborate? What is the issue there? What needs fixing?

offline section containing vmemmap will be lost during hibernation cycle
IIU the above correctly.
-- 
Michal Hocko
SUSE Labs