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,

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]

I was about to give it a new respin now that thw hwpoison stuff has been 
settled.

[1] https://patchwork.kernel.org/cover/11059175/