On Thu 01-08-19 09:50:29, David Hildenbrand wrote: > 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. Sorry, my statement was imprecise. What I meant is what we have documented: * PG_reserved is set for special pages. The "struct page" of such a page * should in general not be touched (e.g. set dirty) except by its owner. the owner part is important. -- Michal Hocko SUSE Labs
