Re: [RFC PATCH 0/5] mm, memory_hotplug: allocate memmap from hotadded memory

[Gerald Schaefer]

On Mon, 31 Jul 2017 14:53:19 +0200
Michal Hocko <mhocko@xxxxxxxxxx> wrote:

> On Mon 31-07-17 14:35:21, Gerald Schaefer wrote:
> > On Fri, 28 Jul 2017 14:19:41 +0200
> > Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> > 
> > > On Thu 27-07-17 08:56:52, Michal Hocko wrote:
> > > > On Wed 26-07-17 17:06:59, Jerome Glisse wrote:
> > > > [...]
> > > > > This does not seems to be an opt-in change ie if i am reading patch 3
> > > > > correctly if an altmap is not provided to __add_pages() you fallback
> > > > > to allocating from begining of zone. This will not work with HMM ie
> > > > > device private memory. So at very least i would like to see some way
> > > > > to opt-out of this. Maybe a new argument like bool forbid_altmap ?
> > > > 
> > > > OK, I see! I will think about how to make a sane api for that.
> > > 
> > > This is what I came up with. s390 guys mentioned that I cannot simply
> > > use the new range at this stage yet. This will need probably some other
> > > changes but I guess we want an opt-in approach with an arch veto in general.
> > > 
> > > So what do you think about the following? Only x86 is update now and I
> > > will split it into two parts but the idea should be clear at least.
> > 
> > This looks good, and the kernel will also boot again on s390 when applied
> > on top of the other 5 patches (plus adding the s390 part here).
> 
> Thanks for testing Gerald! I am still undecided whether the arch code
> should veto MHP_RANGE_ACCESSIBLE if it cannot be supported or just set
> it when it is supported. My last post did the later but the first one
> sounds like a more clear API to me. I will keep thinking about it.
> 
> Anyway, did you have any chance to consider mapping the new physical
> memory range inside arch_add_memory rather than during online on s390?

Well, it still looks like we cannot do w/o splitting up add_memory():
1) (only) set up section map during our initial memory detection, w/o
allocating struct pages, so that the sysfs entries get created also for
our offline memory (or else we have no way to online it later)
2) set up vmemmap and allocate struct pages with your new altmap approach
during our MEM_GOING_ONLINE callback, because only now the memory is really
accessible

Besides the obvious problem that this would need a new interface, there is
also the problem that (at least) show_valid_zones() in drivers/base/memory.c
operates on struct pages from _offline_ memory, for its page_zone() checks.
This will not work well if we have no struct pages for offline memory ...

BTW, the latter may also be a issue with your rework on any architecture.
Not sure if I understood it correctly, but the situation on s390 (i.e.
having offline memory blocks visible in sysfs) should be similar to
the scenario on x86, when you plug in memory, set it online in the acpi
handler, and then manually set it offline again via sysfs. Now the
memory is still visible in sysfs, and reading the valid_zones attribute
will trigger an access to struct pages for that memory. What if this
memory is now physically removed, in a race with such a struct page
access? Before your patch that would probably be OK, because the struct
pages were not part of the removed memory, but now you should get an
addressing exception, right?

Regards,
Gerald