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Date:   Tue, 5 Jan 2021 09:00:57 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     Dan Williams <dan.j.williams@intel.com>, linux-mm@kvack.org,
        LKML <linux-kernel@vger.kernel.org>,
        Oscar Salvador <osalvador@suse.de>
Subject: Re: uninitialized pmem struct pages
Message-ID: <20210105080057.GT13207@dhcp22.suse.cz>
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On Mon 04-01-21 16:44:52, David Hildenbrand wrote:
> On 04.01.21 16:43, David Hildenbrand wrote:
> > On 04.01.21 16:33, Michal Hocko wrote:
> >> On Mon 04-01-21 16:15:23, David Hildenbrand wrote:
> >>> On 04.01.21 16:10, Michal Hocko wrote:
> >> [...]
> >>> Do the physical addresses you see fall into the same section as boot
> >>> memory? Or what's around these addresses?
> >>
> >> Yes I am getting a garbage for the first struct page belonging to the
> >> pmem section [1]
> >> [    0.020161] ACPI: SRAT: Node 0 PXM 0 [mem 0x100000000-0x603fffffff]
> >> [    0.020163] ACPI: SRAT: Node 4 PXM 4 [mem 0x6060000000-0x11d5fffffff] non-volatile
> >>
> >> The pfn without the initialized struct page is 0x6060000. This is a
> >> first pfn in a section.
> > 
> > Okay, so we're not dealing with the "early section" mess I described,
> > different story.
> > 
> > Due to [1], is_mem_section_removable() called
> > pfn_to_page(PHYS_PFN(0x6060000)). page_zone(page) made it crash, as not
> > initialized.
> > 
> > Let's assume this is indeed a reserved pfn in the altmap. What's the
> > actual address of the memmap?
> > 
> > I do wonder what hosts pfn_to_page(PHYS_PFN(0x6060000)) - is it actually
> > part of the actual altmap (i.e. > 0x6060000) or maybe even self-hosted?
> > 
> > If it's not self-hosted, initializing the relevant memmaps should work
> > just fine I guess. Otherwise things get more complicated.
> 
> Oh, I forgot: pfn_to_online_page() should at least in your example make
> sure other pfn walkers are safe. It was just an issue of
> is_mem_section_removable().

Hmm, I suspect you are right. I haven't put this together, thanks! The memory
section is indeed marked offline so pfn_to_online_page would indeed bail
out:
crash> p (0x6060000>>15)
$3 = 3084
crash> p mem_section[3084/128][3084 & 127]
$4 = {
  section_mem_map = 18446736128020054019,
  usage = 0xffff902dcf956680,
  page_ext = 0x0,
  pad = 0
}
crash> p 18446736128020054019 & (1UL<<2)
$5 = 0

That makes it considerably less of a problem than I thought!

Thanks David!
-- 
Michal Hocko
SUSE Labs