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

[Wei Yang <richard.weiyang@xxxxxxxxxxxxxxxxx>]

On Wed, Sep 16, 2020 at 09:31:21PM +0200, 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.

While everything changes with shuffle.

>
>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/
>> 

-- 
Wei Yang
Help you, Help me