Re: [PATCH 6/8] s390/mm: implement MEM_PHYS_ONLINE MEM_PHYS_OFFLINE memory notifiers

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[catching up on mails]

This new approach has the advantage that we do not need to allocate any
additional memory during online phase, neither for direct mapping page
tables nor struct pages, so that memory hotplug can never fail.

Right, just like any other architecture that (triggered by whatever
mechanism) ends up calling add_memory() and friends.

Just for better understanding, are page tables for identity and also
vmemmap mapping not allocated from system memory on other archs? I.e.
no altmap support for that, only for struct pages (so far)?

Yes, only the actual "memmap ("struct page")" comes from altmap space, everything else comes from the buddy during memory hotplug.

The old approach (without altmap) is already a hack, because we add
the memmap / struct pages, but for memory that is not really accessible.

Yes, it's disgusting. And you still allocate other things like memory
block devices or the identify map.

I would say it is special :-). And again, for understanding, all other


things apart from struct pages, still would need to be allocated from
system memory on other archs?


Of course, struct pages would be by far the biggest part, so having
altmap support for that helps a lot. Doing the other allocations also
via altmap would feel natural, but it is not possible yet, or did we
miss something?

The tricky part is making sure ahead of time that that we set aside the required number of pageblocks, to properly control during memory onlining what to set aside and what to expose to the buddy.

See mhp_supports_memmap_on_memory() / memory_block_memmap_on_memory_pages() for the dirty details :)

And with all the disadvantage of pre-allocating struct pages from system

Jep. It never should have been done like that.

At that time, it gave the benefit over all others, that we do not need
to allocate struct pages from system memory, at the time of memory online,
when memory pressure might be high and such allocations might fail.

Agreed. Having the memmap already around can be helpful. But not for all standby memory, that's just pure waste.

... but as memory onlining is triggered by user space, it's likely that that user space cannot even make progress (e.g., start a process to set memory online) to actually trigger memory onlining in serious low-memory situations.

I guess you can say that it should have been done "right" at that time,
e.g. by already adding something like altmap support, instead of our own

Probably yes. IMHO, relying on the existing memory block interface was the low hanging fruit. Now, s390x is just special.

The new approach allows to better integrate s390 to the existing
interface, and also make use of altmap support, which would eliminate
the major disadvantage of the old behaviour. So from s390 perspective,
this new mechanism would be preferred, provided that there is no
functional issue with the "added memory blocks without a memmap"

It achieves that by s390x specific hacks in common code :) Instead of
everybody else that simply uses add_memory() and friends.

Do you see any functional issues, e.g. conflict with common

I don't see functional issues right now, just the way it is done to
implement support for a new feature is a hack IMHO. Replacing hack #1 by
hack #2 is not really something reasonable. Let's try to remove hacks.

Ok, sounds reasonable, let's try that. Introducing some new s390-specific
interface also feels a bit hacky, or ugly, but we'll see if we find a
way so that it is only "special" :-)

As proposed in my other mail, I think there are ways to make s390x happy first and look into a cleaner approach long-term.

Reminds me a bit of that "probe" attribute, that also was an arch-specific
hack initially, IIRC, and is now to be deprecated...

Yeah, that was for interfaces where the kernel has absolutely no clue where/what/how memory gets hotplugged. ARM64 without ACPI.

s390x is completely different though: you know exactly which standby memory exists, where it resides, in which granularity in can be enabled/disabled, ... you don't have to play dangerous "I'm pretty sure there is memory out there although nobody can check so I crash the kernel" games.


David / dhildenb

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