Re: Boot failures with "mm/sparse: Remove CONFIG_SPARSEMEM_ALLOC_MEM_MAP_TOGETHER" on powerpc (was Re: mmotm 2018-07-10-16-50 uploaded)

Hi Michael,

On 07/11/18 at 10:49pm, Michael Ellerman wrote:

akpm@linux-foundation.org writes:

quoted

The mm-of-the-moment snapshot 2018-07-10-16-50 has been uploaded to

   http://www.ozlabs.org/~akpm/mmotm/

...

quoted

* mm-sparse-add-a-static-variable-nr_present_sections.patch
* mm-sparsemem-defer-the-ms-section_mem_map-clearing.patch
* mm-sparsemem-defer-the-ms-section_mem_map-clearing-fix.patch
* mm-sparse-add-a-new-parameter-data_unit_size-for-alloc_usemap_and_memmap.patch
* mm-sparse-optimize-memmap-allocation-during-sparse_init.patch
* mm-sparse-optimize-memmap-allocation-during-sparse_init-checkpatch-fixes.patch

quoted

* mm-sparse-remove-config_sparsemem_alloc_mem_map_together.patch

This seems to be breaking my powerpc pseries qemu boots.

The boot log with some extra debug shows eg:

  $ make pseries_le_defconfig
  $ qemu-system-ppc64 -nographic -vga none -M pseries -m 2G -kernel vmlinux 
  vmemmap_populate f000000000000000..f000000000024000, node 0
        * f000000000000000..f000000001000000 allocated at c000000076000000
  hash__vmemmap_create_mapping: start 0xf000000000000000 size 0x1000000 phys 0x76000000
  hash__vmemmap_create_mapping: failed -1

  <repeated many times>

Then there's lots of other warnings about bad page states and eventually
a NULL deref and we panic().


The problem seems to be that we're calling down into
hash__vmemmap_create_mapping() for every call to vmemmap_populate(),
whereas previously we would only call hash__vmemmap_create_mapping()
once because our vmemmap_populated() would return true.

There's actually a comment in sparse_init() that says:

	 * powerpc need to call sparse_init_one_section right after each
	 * sparse_early_mem_map_alloc, so allocate usemap_map at first.

So changing that behaviour does seem to be the problem.

I assume that comment is talking about the fact that we use pfn_valid()
in vmemmap_populated().

I'm not clear on how to fix it though.

Have you tried reverting that patch and building kernel to test again?
Does it work?