Re: [PATCH RFC 0/4] mm: place pages to the freelist tail when onling and undoing isolation

From: Vlastimil Babka
Date: Wed Sep 23 2020 - 10:31:30 EST

On 9/16/20 9:31 PM, David Hildenbrand wrote:
>> Am 16.09.2020 um 20:50 schrieb osalvador@xxxxxxx:
>> On 2020-09-16 20:34, David Hildenbrand wrote:
>>> When adding separate memory blocks via add_memory*() and onlining them
>>> immediately, the metadata (especially the memmap) of the next block will be
>>> placed onto one of the just added+onlined block. This creates a chain
>>> of unmovable allocations: If the last memory block cannot get
>>> offlined+removed() so will all dependant ones. We directly have unmovable
>>> allocations all over the place.
>>> This can be observed quite easily using virtio-mem, however, it can also
>>> be observed when using DIMMs. The freshly onlined pages will usually be
>>> placed to the head of the freelists, meaning they will be allocated next,
>>> turning the just-added memory usually immediately un-removable. The
>>> fresh pages are cold, prefering to allocate others (that might be hot)
>>> also feels to be the natural thing to do.
>>> It also applies to the hyper-v balloon xen-balloon, and ppc64 dlpar: when
>>> adding separate, successive memory blocks, each memory block will have
>>> unmovable allocations on them - for example gigantic pages will fail to
>>> allocate.
>>> While the ZONE_NORMAL doesn't provide any guarantees that memory can get
>>> offlined+removed again (any kind of fragmentation with unmovable
>>> allocations is possible), there are many scenarios (hotplugging a lot of
>>> memory, running workload, hotunplug some memory/as much as possible) where
>>> we can offline+remove quite a lot with this patchset.
>> Hi David,
> Hi Oscar.
>> I did not read through the patchset yet, so sorry if the question is nonsense, but is this not trying to fix the same issue the vmemmap patches did? [1]
> Not nonesense at all. It only helps to some degree, though. It solves the dependencies due to the memmap. However, it‘s not completely ideal, especially for single memory blocks.
> With single memory blocks (virtio-mem, xen-balloon, hv balloon, ppc dlpar) you still have unmovable (vmemmap chunks) all over the physical address space. Consider the gigantic page example after hotplug. You directly fragmented all hotplugged memory.
> Of course, there might be (less extreme) dependencies due page tables for the identity mapping, extended struct pages and similar.
> Having that said, there are other benefits when preferring other memory over just hotplugged memory. Think about adding+onlining memory during boot (dimms under QEMU, virtio-mem), once the system is up you will have most (all) of that memory completely untouched.
> So while vmemmap on hotplugged memory would tackle some part of the issue, there are cases where this approach is better, and there are even benefits when combining both.

I see the point, but I don't think the head/tail mechanism is great for this. It
might sort of work, but with other interfering activity there are no guarantees
and it relies on a subtle implementation detail. There are better mechanisms
possible I think, such as preparing a larger MIGRATE_UNMOVABLE area in the
existing memory before we allocate those long-term management structures. Or
onlining a bunch of blocks as zone_movable first and only later convert to
zone_normal in a controlled way when existing normal zone becomes depeted?

I guess it's an issue that the e.g. 128M block onlines are so disconnected from
each other it's hard to employ a strategy that works best for e.g. a whole bunch
of GB onlined at once. But I noticed some effort towards new API, so maybe that
will be solved there too?

> Thanks!
> David
>> I was about to give it a new respin now that thw hwpoison stuff has been settled.
>> [1]