[PATCH v1 1/1] mm: buddy page accessed before initialized

[Pavel Tatashin]

This problem is seen when machine is rebooted after kexec:
A message like this is printed:
==========================================================================
WARNING: CPU: 21 PID: 249 at linux/lib/list_debug.c:53__listd+0x83/0xa0
Modules linked in:
CPU: 21 PID: 249 Comm: pgdatinit0 Not tainted 4.14.0-rc6_pt_deferred #90
Hardware name: Oracle Corporation ORACLE SERVER X6-2/ASM,MOTHERBOARD,1U,
BIOS 3016
node 1 initialised, 32444607 pages in 1679ms
task: ffff880180e75a00 task.stack: ffffc9000cdb0000
RIP: 0010:__list_del_entry_valid+0x83/0xa0
RSP: 0000:ffffc9000cdb3d18 EFLAGS: 00010046
RAX: 0000000000000054 RBX: 0000000000000009 RCX: ffffffff81c5f3e8
RDX: 0000000000000000 RSI: 0000000000000086 RDI: 0000000000000046
RBP: ffffc9000cdb3d18 R08: 00000000fffffffe R09: 0000000000000154
R10: 0000000000000005 R11: 0000000000000153 R12: 0000000001fcdc00
R13: 0000000001fcde00 R14: ffff88207ffded00 R15: ffffea007f370000
FS:  0000000000000000(0000) GS:ffff881fffac0000(0000) knlGS:0
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000000 CR3: 000000407ec09001 CR4: 00000000003606e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 free_one_page+0x103/0x390
 __free_pages_ok+0x1cf/0x2d0
 __free_pages+0x19/0x30
 __free_pages_boot_core+0xae/0xba
 deferred_free_range+0x60/0x94
 deferred_init_memmap+0x324/0x372
 kthread+0x109/0x140
 ? __free_pages_bootmem+0x2e/0x2e
 ? kthread_park+0x60/0x60
 ret_from_fork+0x25/0x30

list_del corruption. next->prev should be ffffea007f428020, but was
ffffea007f1d8020
==========================================================================

The problem happens in this path:

page_alloc_init_late
  deferred_init_memmap
    deferred_init_range
      __def_free
        deferred_free_range
          __free_pages_boot_core(page, order)
            __free_pages()
              __free_pages_ok()
                free_one_page()
                  __free_one_page(page, pfn, zone, order, migratetype);

deferred_init_range() initializes one page at a time by calling
__init_single_page(), once it initializes pageblock_nr_pages pages, it
calls deferred_free_range() to free the initialized pages to the buddy
allocator. Eventually, we reach __free_one_page(), where we compute buddy
page:
	buddy_pfn = __find_buddy_pfn(pfn, order);
	buddy = page + (buddy_pfn - pfn);

buddy_pfn is computed as pfn ^ (1 << order), or pfn + pageblock_nr_pages.
Thefore, buddy page becomes a page one after the range that currently was
initialized, and we access this page in this function. Also, later when we
return back to deferred_init_range(), the buddy page is initialized again.

So, in order to avoid this issue, we must initialize the buddy page prior
to calling deferred_free_range().

Signed-off-by: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>
---
 mm/page_alloc.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 97687b38da05..f3ea06db3eed 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1500,9 +1500,17 @@ static unsigned long deferred_init_range(int nid, int zid, unsigned long pfn,
 			__init_single_page(page, pfn, zid, nid);
 			nr_free++;
 		} else {
-			nr_pages += __def_free(&nr_free, &free_base_pfn, &page);
 			page = pfn_to_page(pfn);
 			__init_single_page(page, pfn, zid, nid);
+			/*
+			 * We must free previous range after initializing the
+			 * first page of the next range. This is because first
+			 * page may be accessed in __free_one_page(), when buddy
+			 * page is computed:
+			 *   buddy_pfn = pfn + pageblock_nr_pages
+			 */
+			deferred_free_range(free_base_pfn, nr_free);
+			nr_pages += nr_free;
 			free_base_pfn = pfn;
 			nr_free = 1;
 			cond_resched();
-- 
2.14.3