mm, memory_hotplug: initialize struct pages for the full memory section

From: Mikhail Zaslonko <[email protected]>
Subject: mm, memory_hotplug: initialize struct pages for the full memory section

If memory end is not aligned with the sparse memory section boundary, the mapping of such a section is only partly initialized. This may lead to VM_BUG_ON due to uninitialized struct page access from is_mem_section_removable() or test_pages_in_a_zone() function triggered by memory_hotplug sysfs handlers:

Here are the the panic examples:
CONFIG_DEBUG_VM=y
CONFIG_DEBUG_VM_PGFLAGS=y

kernel parameter mem=2050M
--------------------------
page:000003d082008000 is uninitialized and poisoned

 page dumped because: VM_BUG_ON_PAGE(PagePoisoned(p))
 Call Trace:
 ([<0000000000385b26>] test_pages_in_a_zone+0xde/0x160)
  [<00000000008f15c4>] show_valid_zones+0x5c/0x190
  [<00000000008cf9c4>] dev_attr_show+0x34/0x70
  [<0000000000463ad0>] sysfs_kf_seq_show+0xc8/0x148
  [<00000000003e4194>] seq_read+0x204/0x480
  [<00000000003b53ea>] __vfs_read+0x32/0x178
  [<00000000003b55b2>] vfs_read+0x82/0x138
  [<00000000003b5be2>] ksys_read+0x5a/0xb0
  [<0000000000b86ba0>] system_call+0xdc/0x2d8
 Last Breaking-Event-Address:
  [<0000000000385b26>] test_pages_in_a_zone+0xde/0x160
 Kernel panic - not syncing: Fatal exception: panic_on_oops

kernel parameter mem=3075M
--------------------------
page:000003d08300c000 is uninitialized and poisoned

 page dumped because: VM_BUG_ON_PAGE(PagePoisoned(p))
 Call Trace:
 ([<000000000038596c>] is_mem_section_removable+0xb4/0x190)
  [<00000000008f12fa>] show_mem_removable+0x9a/0xd8
  [<00000000008cf9c4>] dev_attr_show+0x34/0x70
  [<0000000000463ad0>] sysfs_kf_seq_show+0xc8/0x148
  [<00000000003e4194>] seq_read+0x204/0x480
  [<00000000003b53ea>] __vfs_read+0x32/0x178
  [<00000000003b55b2>] vfs_read+0x82/0x138
  [<00000000003b5be2>] ksys_read+0x5a/0xb0
  [<0000000000b86ba0>] system_call+0xdc/0x2d8
 Last Breaking-Event-Address:
  [<000000000038596c>] is_mem_section_removable+0xb4/0x190
 Kernel panic - not syncing: Fatal exception: panic_on_oops

Fix the problem by initializing the last memory section of each zone in memmap_init_zone() till the very end, even if it goes beyond the zone end.

Michal said:

: This has alwways been problem AFAIU. It just went unnoticed because we : have zeroed memmaps during allocation before f7f99100d8d9 ("mm: stop : zeroing memory during allocation in vmemmap") and so the above test : would simply skip these ranges as belonging to zone 0 or provided a : garbage.
:
: So I guess we do care for post f7f99100d8d9 kernels mostly and : therefore Fixes: f7f99100d8d9 ("mm: stop zeroing memory during : allocation in vmemmap")

---

 mm/page_alloc.c |   12 ++++++++++++
 1 file changed, 12 insertions(+)

@@ -5542,6 +5542,18 @@ void __meminit memmap_init_zone(unsigned
 			cond_resched();
 		}
 	}
+#ifdef CONFIG_SPARSEMEM
+	/*
+	 * If the zone does not span the rest of the section then
+	 * we should at least initialize those pages. Otherwise we
+	 * could blow up on a poisoned page in some paths which depend
+	 * on full sections being initialized (e.g. memory hotplug).
+	 */
+	while (end_pfn % PAGES_PER_SECTION) {
+		__init_single_page(pfn_to_page(end_pfn), end_pfn, zone, nid);
+		end_pfn++;
+	}
+#endif
 }
 
 #ifdef CONFIG_ZONE_DEVICE

_