mm: don't allow deferred pages with NEED_PER_CPU_KM

It is unsafe to do virtual to physical translations before mm_init() is
called if struct page is needed in order to determine the memory section
number (see SECTION_IN_PAGE_FLAGS).  This is because only in mm_init()
we initialize struct pages for all the allocated memory when deferred
struct pages are used.

My recent fix in commit c9e97a1 ("mm: initialize pages on demand
during boot") exposed this problem, because it greatly reduced number of
pages that are initialized before mm_init(), but the problem existed
even before my fix, as Fengguang Wu found.

Below is a more detailed explanation of the problem.

We initialize struct pages in four places:

1. Early in boot a small set of struct pages is initialized to fill the
   first section, and lower zones.

2. During mm_init() we initialize "struct pages" for all the memory that
   is allocated, i.e reserved in memblock.

3. Using on-demand logic when pages are allocated after mm_init call
   (when memblock is finished)

4. After smp_init() when the rest free deferred pages are initialized.

The problem occurs if we try to do va to phys translation of a memory
between steps 1 and 2.  Because we have not yet initialized struct pages
for all the reserved pages, it is inherently unsafe to do va to phys if
the translation itself requires access of "struct page" as in case of
this combination: CONFIG_SPARSE && !CONFIG_SPARSE_VMEMMAP

The following path exposes the problem:

  start_kernel()
   trap_init()
    setup_cpu_entry_areas()
     setup_cpu_entry_area(cpu)
      get_cpu_gdt_paddr(cpu)
       per_cpu_ptr_to_phys(addr)
        pcpu_addr_to_page(addr)
         virt_to_page(addr)
          pfn_to_page(__pa(addr) >> PAGE_SHIFT)

We disable this path by not allowing NEED_PER_CPU_KM with deferred
struct pages feature.

The problems are discussed in these threads:
  http://lkml.kernel.org/r/[email protected]
  http://lkml.kernel.org/r/[email protected]
  http://lkml.kernel.org/r/[email protected]

Link: http://lkml.kernel.org/r/[email protected]
Fixes: 3a80a7f ("mm: meminit: initialise a subset of struct pages if CONFIG_DEFERRED_STRUCT_PAGE_INIT is set")
Signed-off-by: Pavel Tatashin <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Reviewed-by: Andrew Morton <[email protected]>
Cc: Steven Sistare <[email protected]>
Cc: Daniel Jordan <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Fengguang Wu <[email protected]>
Cc: Dennis Zhou <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: Pavel Tatashin

mm/Kconfig

@@ -636,6 +636,7 @@ config DEFERRED_STRUCT_PAGE_INIT
 	default n
 	depends on NO_BOOTMEM
 	depends on !FLATMEM
+	depends on !NEED_PER_CPU_KM
 	help
 	  Ordinarily all struct pages are initialised during early boot in a
 	  single thread. On very large machines this can take a considerable