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

[David Hildenbrand]

On 25.03.21 16:12, Michal Hocko wrote:
> 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?

AFAIU, only the content of the memory block won't be restored - whatever 
memory content existed before the restore operation is kept.

The structures that define how the vmemmap should look like - the memory 
sections and the page tables used for describing the vmemmap should 
properly get saved+restored, as these are located on online memory.

So the vmemmap layout should look after restoring just like before saving.

--
Thanks,

David / dhildenb