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

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Message-ID: <20190801072430.GF11627@dhcp22.suse.cz>
Date:   Thu, 1 Aug 2019 09:24:30 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     Rashmica Gupta <rashmica.g@...il.com>,
        Oscar Salvador <osalvador@...e.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dan Williams <dan.j.williams@...el.com>,
        pasha.tatashin@...een.com, Jonathan.Cameron@...wei.com,
        anshuman.khandual@....com, Vlastimil Babka <vbabka@...e.cz>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 0/5] Allocate memmap from hotadded memory

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