[PATCH] mm: fix pfn spans two sections in has_unmovable_pages()

[Xishi Qiu]

If the pfn is not aligned to pageblock, the check pfn may access a next
pageblcok, and the next pageblock may belong to a next section. Because
struct page has not been alloced in the next section, so kernel panic. 

I find the caller of has_unmovable_pages() has passed a aligned pfn, so it
doesn't have this problem. But the earlier kernel version(e.g. v3.10) has.
e.g. echo xxx > /sys/devices/system/memory/soft_offline_page could trigger
it. The following log is from RHEL v7.1

[14111.611492] Stack:
[14111.611494] ffffffff8115d952 0000000000000000 01ff880c393ebe40 ffff880c7ffd9000
[14111.611500] ffffea0061ffffc0 ffff880c7ffd9068 0000000000000286 0000000000000001
[14111.611505] ffff880c393ebe10 ffffffff811c265a 000000000187ffff 0000000000000200
[14111.611511] Call Trace:
[14111.611516] [<ffffffff8115d952>] ? has_unmovable_pages+0xd2/0x130
[14111.611521] [<ffffffff811c265a>] set_migratetype_isolate+0xda/0x170
[14111.611526] [<ffffffff811c187a>] soft_offline_page+0x9a/0x590
[14111.611530] [<ffffffff812e7cab>] ? _kstrtoull+0x3b/0xa0
[14111.611535] [<ffffffff813e158f>] store_soft_offline_page+0xaf/0xf0
[14111.611539] [<ffffffff813cae18>] dev_attr_store+0x18/0x30
[14111.611544] [<ffffffff8123c046>] sysfs_write_file+0xc6/0x140
[14111.611548] [<ffffffff811c5b5d>] vfs_write+0xbd/0x1e0
[14111.611551] [<ffffffff811c65a8>] SyS_write+0x58/0xb0
[14111.611556] [<ffffffff8160f509>] system_call_fastpath+0x16/0x1b
[14111.611559] Code: 66 66 66 90 48 83 e0 fd 0c a0 5d c3 66 2e 0f 1f 84 00 00 00 00 00 48 89 f8 66 66 66 90 48 83 c8 42 0c a0 5d c3 90 66 66 66 66 90 <8b> 07 25 00 c0 00 00 75 02 f3 c3 48 8b 07 f6 c4 80 75 0f 48 81
[14111.611594] RIP [<ffffffff81199fc5>] PageHuge+0x5/0x40
[14111.611598] RSP <ffff880c393ebd80>
[14111.611600] CR2: ffffea0062000000
[14111.611604] ---[ end trace 9f780ed1def334c6 ]---
[14111.678586] Kernel panic - not syncing: Fatal exception

Signed-off-by: Xishi Qiu <qiuxishi@xxxxxxxxxx>
---
 mm/page_alloc.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 59de90d..9afc1bc 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -6842,6 +6842,7 @@ bool has_unmovable_pages(struct zone *zone, struct page *page, int count,
 		return false;
 
 	pfn = page_to_pfn(page);
+	pfn = pfn & ~(pageblock_nr_pages - 1);
 	for (found = 0, iter = 0; iter < pageblock_nr_pages; iter++) {
 		unsigned long check = pfn + iter;
 
-- 
1.8.3.1