linux-kernel - Re: uninitialized pmem struct pages

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Date:   Mon, 4 Jan 2021 21:33:06 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     David Hildenbrand <david@...hat.com>,
        Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Oscar Salvador <osalvador@...e.de>
Subject: Re: uninitialized pmem struct pages

On Mon, Jan 4, 2021 at 7:59 AM Michal Hocko <mhocko@...e.com> wrote:
>
> 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

Does /proc/iomem show an active namespace in the range? You should be
able to skip ahead to the first pfn in that namespace to find the
first dev_pagemap. I would have expected pfn_to_online_page() to have
saved you here. This address range is section aligned.