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

[David Hildenbrand <david@xxxxxxxxxx>]

> 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.

Thanks!

David

> 
> I was about to give it a new respin now that thw hwpoison stuff has been 
> settled.
> 
> [1] https://patchwork.kernel.org/cover/11059175/
>