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] https://patchwork.kernel.org/cover/11059175/ >> >
