On 01.08.19 09:26, David Hildenbrand wrote: > On 01.08.19 09:24, Michal Hocko wrote: >> On Thu 01-08-19 09:18:47, David Hildenbrand wrote: >>> On 01.08.19 09:17, Michal Hocko wrote: >>>> On Thu 01-08-19 09:06:40, Rashmica Gupta wrote: >>>>> On Wed, 2019-07-31 at 14:08 +0200, Michal Hocko wrote: >>>>>> On Tue 02-07-19 18:52:01, Rashmica Gupta wrote: >>>>>> [...] >>>>>>>> 2) Why it was designed, what is the goal of the interface? >>>>>>>> 3) When it is supposed to be used? >>>>>>>> >>>>>>>> >>>>>>> There is a hardware debugging facility (htm) on some power chips. >>>>>>> To use >>>>>>> this you need a contiguous portion of memory for the output to be >>>>>>> dumped >>>>>>> to - and we obviously don't want this memory to be simultaneously >>>>>>> used by >>>>>>> the kernel. >>>>>> >>>>>> How much memory are we talking about here? Just curious. >>>>> >>>>> From what I've seen a couple of GB per node, so maybe 2-10GB total. >>>> >>>> OK, that is really a lot to keep around unused just in case the >>>> debugging is going to be used. >>>> >>>> I am still not sure the current approach of (ab)using memory hotplug is >>>> ideal. Sure there is some overlap but you shouldn't really need to >>>> offline the required memory range at all. All you need is to isolate the >>>> memory from any existing user and the page allocator. Have you checked >>>> alloc_contig_range? >>>> >>> >>> Rashmica mentioned somewhere in this thread that the virtual mapping >>> must not be in place, otherwise the HW might prefetch some of this >>> memory, leading to errors with memtrace (which checks that in HW). >> >> Does anything prevent from unmapping the pfn range from the direct >> mapping? > > I am not sure about the implications of having > pfn_valid()/pfn_present()/pfn_online() return true but accessing it > results in crashes. (suspend, kdump, whatever other technology touches > online memory) (oneidea: we could of course go ahead and mark the pages PG_offline before unmapping the pfn range to work around these issues) > > (sounds more like a hack to me than just going ahead and > removing/readding the memory via a clean interface we have) > -- Thanks, David / dhildenb
