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

[Igor Mammedov <imammedo@redhat.com>]

On Mon, 31 Jul 2017 19:58:30 +0200
Gerald Schaefer <gerald.schaefer@de.ibm.com> wrote:

> On Mon, 31 Jul 2017 17:53:50 +0200
> Michal Hocko <mhocko@kernel.org> wrote:
> 
> > On Mon 31-07-17 17:04:59, Gerald Schaefer wrote:  
> > > On Mon, 31 Jul 2017 14:53:19 +0200
> > > Michal Hocko <mhocko@kernel.org> wrote:
> > >   
> > > > On Mon 31-07-17 14:35:21, Gerald Schaefer wrote:  
> > > > > On Fri, 28 Jul 2017 14:19:41 +0200
> > > > > Michal Hocko <mhocko@kernel.org> 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  
> > 
> > As I've tried to mentioned in my other response. This is not possible
> > because there are memory hotplug usecases which never do an explicit
> > online.  
> 
> Of course the default behaviour should not change, we only need an option
> to do the "2-stage-approach". E.g. we would call __add_pages() from our
> MEM_GOING_ONLINE handler, and not from arch_add_memory() as before, but
> then we would need some way to add memory sections (for generating sysfs
> memory blocks) only, without allocating struct pages. See also below.
> 
> > 
> > I am sorry to ask again. But why exactly cannot we make the range
> > accessible from arch_add_memory on s390?  
> 
> We have no acpi or other events to indicate new memory, both online and
> offline memory needs to be (hypervisor) defined upfront, and then we want
> to be able to use memory hotplug for ballooning during runtime.
> 
> Making the range accessible is equivalent to a hypervisor call that assigns
> the memory to the guest. The problem with arch_add_memory() is now that
> this gets called from add_memory(), which we call during initial memory
> detection for the offline memory ranges. At that time, assigning all
> offline memory to the guest, and thus making it accessible, would break
> the ballooning usecase (even if it is still offline in Linux, the
> hypervisor could not use it for other guests any more).
> 
> The main difference to other architectures is that we can not simply
> call add_memory() (and thus arch_add_memory()) at the time when the
> offline memory is actually supposed to get online (e.g. triggered by acpi).
> We rather need to somehow make sure that the offline memory is detected
> early, and sysfs entries are created for it, so that it can be set online
> later on demand.
> 
> Maybe our design to use add_memory() for offline ranges during memory
> detection was wrong, or overkill, since we actually only need to establish
> a memory section, if I understood the sysfs code right. But I currently
> see no other way to make sure that we get the sysfs attributes. And of
> course the presence of users that work on offline struct pages, like
> valid_zones, is also not helpful.
> 
> >   
> > > 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 ...  
> > 
> > Yes.
> >   
> > > 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?  
> > 
> > The memmap goes away together with the whole section tear down. And we
> > shouldn't have any users of any struct page by that time. Memblock sysfs
> > should be down as well. I will go and double check whether there are any
> > possible races.  
> 
> I was thinking of someone pulling out a DIMM whose range was (manually)
> set offline before. It looks like (arch_)remove_memory() is not triggered
> directly on setting it offline, but rather by an acpi event, probably after
> physical memory removal.
Order here a little bit different, first and ACPI event sent and processed
by kernel (including removing ranges from sysfs) and only then kernel should
call ACPI _EJ0 method on DIMM device when there shouldn't be any users left
nor races should happen.

> And that would mean that a user could just read
> sysfs valid_zones in a loop, after setting it offline and before the
> physical removal, thereby accessing struct pages in the offline range,
> which would then race with the physical DIMM removal.
> 
> However, as you can see, s390 memory hotplug works in a special way,
> so I may have gotten the wrong picture of how it works on "normal"
> architectures :-)
> 
> Regards,
> Gerald
> 

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