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Date:   Thu, 1 Aug 2019 09:34:07 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     David Hildenbrand <david@redhat.com>
Cc:     Rashmica Gupta <rashmica.g@gmail.com>,
        Oscar Salvador <osalvador@suse.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        Dan Williams <dan.j.williams@intel.com>,
        pasha.tatashin@soleen.com, Jonathan.Cameron@huawei.com,
        anshuman.khandual@arm.com, Vlastimil Babka <vbabka@suse.cz>,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 0/5] Allocate memmap from hotadded memory
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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