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Date:   Mon, 4 Jan 2021 16:59:31 +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: <20210104155931.GN13207@dhcp22.suse.cz>
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On Mon 04-01-21 16:43:49, 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.

Exactly!

> Let's assume this is indeed a reserved pfn in the altmap. What's the
> actual address of the memmap?

Not sure what exactly you are asking for but crash says
crash> kmem -p 6060000
      PAGE          PHYSICAL      MAPPING       INDEX CNT FLAGS
fffff8c600181800     6060000                0        0  0 fffffc0000000
 
> 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?

I am not really familiar with the pmem so I would need more assistance
here. I've tried this (shot into the dark):
crash> struct page.pgmap fffff8c600181800
      pgmap = 0xfffff8c600181808
crash> struct -x dev_pagemap 0xfffff8c600181808
struct dev_pagemap {
  altmap = {
    base_pfn = 0xfffff8c600181808, 
    end_pfn = 0xfffff8c600181808, 
    reserve = 0x0, 
    free = 0x0, 
    align = 0x0, 
    alloc = 0xffffffff
  }, 
  res = {
    start = 0x0, 
    end = 0xfffffc0000000, 
    name = 0xfffff8c600181848 "H\030\030", 
    flags = 0xfffff8c600181848, 
    desc = 0x0, 
    parent = 0x0, 
    sibling = 0x0, 
    child = 0xffffffff
  }, 
  ref = 0x0, 
  internal_ref = {
    count = {
      counter = 0xfffffc0000000
    }, 
    percpu_count_ptr = 0xfffff8c600181888, 
    release = 0xfffff8c600181888, 
    confirm_switch = 0x0, 
    force_atomic = 0x0, 
    allow_reinit = 0x0, 
    rcu = {
      next = 0x0, 
      func = 0xffffffff
    }
  }, 
  done = {
    done = 0x0, 
    wait = {
      lock = {
        {
          rlock = {
            raw_lock = {
              {
                val = {
                  counter = 0xc0000000
                }, 
                {
                  locked = 0x0, 
                  pending = 0x0
                }, 
                {
                  locked_pending = 0x0, 
                  tail = 0xc000
                }
              }
            }
          }
        }
      }, 
      head = {
        next = 0xfffff8c6001818c8, 
        prev = 0xfffff8c6001818c8
      }
    }
  }, 
  dev = 0x0, 
  type = 0, 
  flags = 0x0, 
  ops = 0x0
}

Not sure whether this is of any use.
 
> If it's not self-hosted, initializing the relevant memmaps should work
> just fine I guess. Otherwise things get more complicated.

-- 
Michal Hocko
SUSE Labs