On 01.08.19 09:34, Michal Hocko wrote: > On Thu 01-08-19 09:26:35, 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) > > If those pages are marked as Reserved then nobody should be touching > them anyway. Which is not true as I remember we already discussed - I even documented what PG_reserved can mean after that discussion in page-flags.h (e.g., memmap of boot memory) - that's why we introduced PG_offline after all. -- Thanks, David / dhildenb
