On 5/23/2016 10:37 PM, Joonsoo Kim wrote:
On Fri, May 20, 2016 at 10:00:06AM -0700, Shi, Yang wrote:
On 5/19/2016 7:40 PM, Joonsoo Kim wrote:
2016-05-20 2:18 GMT+09:00 Shi, Yang yang.shi@linaro.org:
On 5/18/2016 5:28 PM, Joonsoo Kim wrote:
Vlastiml, thanks for ccing me on original bug report.
On Wed, May 18, 2016 at 03:23:45PM -0700, Yang Shi wrote:
When enabling the below kernel configs:
CONFIG_DEFERRED_STRUCT_PAGE_INIT CONFIG_DEBUG_PAGEALLOC CONFIG_PAGE_EXTENSION CONFIG_DEBUG_VM
kernel bootup may fail due to the following oops:
BUG: unable to handle kernel NULL pointer dereference at (null) IP: [<ffffffff8118d982>] free_pcppages_bulk+0x2d2/0x8d0 PGD 0 Oops: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC Modules linked in: CPU: 11 PID: 106 Comm: pgdatinit1 Not tainted 4.6.0-rc5-next-20160427 #26 Hardware name: Intel Corporation S5520HC/S5520HC, BIOS S5500.86B.01.10.0025.030220091519 03/02/2009 task: ffff88017c080040 ti: ffff88017c084000 task.ti: ffff88017c084000 RIP: 0010:[<ffffffff8118d982>] [<ffffffff8118d982>] free_pcppages_bulk+0x2d2/0x8d0 RSP: 0000:ffff88017c087c48 EFLAGS: 00010046 RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000001 RDX: 0000000000000980 RSI: 0000000000000080 RDI: 0000000000660401 RBP: ffff88017c087cd0 R08: 0000000000000401 R09: 0000000000000009 R10: ffff88017c080040 R11: 000000000000000a R12: 0000000000000400 R13: ffffea0019810000 R14: ffffea0019810040 R15: ffff88066cfe6080 FS: 0000000000000000(0000) GS:ffff88066cd40000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 0000000000000000 CR3: 0000000002406000 CR4: 00000000000006e0 Stack: ffff88066cd5bbd8 ffff88066cfe6640 0000000000000000 0000000000000000 0000001f0000001f ffff88066cd5bbe8 ffffea0019810000 000000008118f53e 0000000000000009 0000000000000401 ffffffff0000000a 0000000000000001 Call Trace: [<ffffffff8118f602>] free_hot_cold_page+0x192/0x1d0 [<ffffffff8118f69c>] __free_pages+0x5c/0x90 [<ffffffff8262a676>] __free_pages_boot_core+0x11a/0x14e [<ffffffff8262a6fa>] deferred_free_range+0x50/0x62 [<ffffffff8262aa46>] deferred_init_memmap+0x220/0x3c3 [<ffffffff8262a826>] ? setup_per_cpu_pageset+0x35/0x35 [<ffffffff8108b1f8>] kthread+0xf8/0x110 [<ffffffff81c1b732>] ret_from_fork+0x22/0x40 [<ffffffff8108b100>] ? kthread_create_on_node+0x200/0x200 Code: 49 89 d4 48 c1 e0 06 49 01 c5 e9 de fe ff ff 4c 89 f7 44 89 4d b8 4c 89 45 c0 44 89 5d c8 48 89 4d d0 e8 62 c7 07 00 48 8b 4d d0 <48> 8b 00 44 8b 5d c8 4c 8b 45 c0 44 8b 4d b8 a8 02 0f 84 05 ff RIP [<ffffffff8118d982>] free_pcppages_bulk+0x2d2/0x8d0 RSP <ffff88017c087c48> CR2: 0000000000000000
The problem is lookup_page_ext() returns NULL then page_is_guard() tried to access it in page freeing.
page_is_guard() depends on PAGE_EXT_DEBUG_GUARD bit of page extension flag, but freeing page might reach here before the page_ext arrays are allocated when feeding a range of pages to the allocator for the first time during bootup or memory hotplug.
Patch itself looks find to me because I also found that this kind of problem happens during memory hotplug. So, we need to fix more sites, all callers of lookup_page_ext().
Yes, I agree. I will come up with a patch or a couple of patches to check the return value of lookup_page_ext().
But, I'd like to know how your problem occurs during bootup. debug_guardpage_enabled() is turned to 'enable' after page_ext is initialized. Before that, page_is_guard() unconditionally returns false so I think that the problem what you mentioned can't happen.
Could you check that when debug_guardpage_enabled() returns 'enable' and init_section_page_ext() is called?
I think the problem is I have CONFIG_DEFERRED_STRUCT_PAGE_INIT enabled, which will defer some struct pages initialization to "pgdatinitX" kernel thread in page_alloc_init_late(). But, page_ext_init() is called before it. So, it leads debug_guardpage_enabled() return true, but page extension is not allocated yet for the struct pages initialized by "pgdatinitX".
No. After page_ext_init(), it is ensured that all page extension is initialized.
It sounds page_ext_init() should be called after page_alloc_init_late(). Or it should be just incompatible with CONFIG_DEFERRED_STRUCT_PAGE_INIT.
I will try to move the init call around.
We need to investigate more. I guess that problem is introduced by CONFIG_DEFERRED_STRUCT_PAGE_INIT. It makes pfn_to_nid() invalid until page_alloc_init_late() is done. That is a big side-effect. If there is pfn walker and it uses pfn_to_nid() between memmap_init_zone() and page_alloc_init_late(), it also has same problem. So, we need to think how to fix it more carefully.
Thanks for the analysis. I think you are correct. Since pfn_to_nid() depends on memmap which has not been fully setup yet until page_alloc_init_late() is done.
So, for such usecase early_pfn_to_nid() should be used.
Anyway, to make sure that my assumption is true, could you confirm that below change fix your problem?
Yes, it does.
Thanks.
----->8---------- diff --git a/mm/page_ext.c b/mm/page_ext.c index 2d864e6..cac5dc9 100644 --- a/mm/page_ext.c +++ b/mm/page_ext.c @@ -391,7 +391,7 @@ void __init page_ext_init(void) * -------------pfn--------------> * N0 | N1 | N2 | N0 | N1 | N2|.... */
if (pfn_to_nid(pfn) != nid)
if (!early_pfn_in_nid(pfn nid))early_pfn_in_nid() is static function in page_alloc.c. I'm supposed early_pfn_to_nid() should be used.
Thanks for checking. Then, please revert your patch "mm: call page_ext_init() after all struct pages are initialized" and apply this change, because deferring page_ext_init() would make page owner which uses page_ext miss some early page allocation callsites. Although it already miss some early page allocation callsites, we don't need to miss more.
Definitely agree. I will come up with an incremental patch to adopt the new solution.
Thanks, Yang
Thanks.