Re: uninitialized pmem struct pages

[Dan Williams]

On Tue, Jan 5, 2021 at 12:17 AM Michal Hocko <mhocko@xxxxxxxx> wrote:
>
> On Tue 05-01-21 09:01:00, Michal Hocko wrote:
> > 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!
>
> Forgot to add that those who are running kernels without 53cdc1cb29e8
> ("drivers/base/memory.c: indicate all memory blocks as removable") for
> some reason can fix the crash by the following simple patch.
>
> Index: linux-5.3-users_mhocko_SLE15-SP2_for-next/drivers/base/memory.c
> ===================================================================
> --- linux-5.3-users_mhocko_SLE15-SP2_for-next.orig/drivers/base/memory.c
> +++ linux-5.3-users_mhocko_SLE15-SP2_for-next/drivers/base/memory.c
> @@ -152,9 +152,14 @@ static ssize_t removable_show(struct dev
>                 goto out;
>
>         for (i = 0; i < sections_per_block; i++) {
> -               if (!present_section_nr(mem->start_section_nr + i))
> +               unsigned long nr = mem->start_section_nr + i;
> +               if (!present_section_nr(nr))
>                         continue;
> -               pfn = section_nr_to_pfn(mem->start_section_nr + i);
> +               if (!online_section_nr()) {

I assume that's onlince_section_nr(nr) in the version that compiles?

This makes sense because the memory block size is larger than the
section size. I suspect you have 1GB memory block size on this system,
but since the System RAM and PMEM collide at a 512MB alignment in a
memory block you end up walking the back end of the last 512MB of the
System RAM memory block and run into the offline PMEM section.

So, I don't think it's pfn_to_online_page that necessarily needs to
know how to disambiguate each page, it's things that walk sections and
memory blocks and expects them to be consistent over the span.