Re: [PATCH v2 0/5] Allocate memmap from hotadded memory

[Michal Hocko]

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