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? -- Michal Hocko SUSE Labs
