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. > (sounds more like a hack to me than just going ahead and > removing/readding the memory via a clean interface we have) Right, but the interface that we have is quite restricted in what it can really offline. -- Michal Hocko SUSE Labs
