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

From: Michal Hocko <mhocko@suse.com>
To: David Hildenbrand <david@redhat.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: Thu, 25 Mar 2021 16:12:19 +0100	[thread overview]
Message-ID: <YFyoU/rkEPK3VPlN@dhcp22.suse.cz> (raw)
In-Reply-To: <31c3e6f7-f631-7b00-2c33-518b0f24a75f@redhat.com>

On Thu 25-03-21 15:46:22, David Hildenbrand wrote:
> On 25.03.21 15:34, Michal Hocko wrote:
> > 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.
> > 
> 
> Can tell me how that is a problem with Oscars current patch? I only see this
> being a problem with what you propose - most probably I am missing something
> important here.
> 
> Offline memory sections don't have a valid memmap (assumption: garbage). On
> hibernation, the whole offline memory block won't be saved, including the
> vmemmap content that resides on the block. This includes the vmemmap of the
> vmemmap pages, which is itself.
> 
> When restoring, the whole memory block will contain garbage, including the
> whole vmemmap - which is marked to be offline and to contain garbage.

Hmm, so I might be misunderstanding the restoring part. But doesn't that
mean that the whole section/memory block won't get restored because it
is offline and therefore the vmemmap would be pointing to nowhere?
-- 
Michal Hocko
SUSE Labs