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) (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
