[+CC Joonsoo based on git blame] On 05/05/2016 11:13 PM, Shi, Yang wrote:
Hi folks, When I enable the below kernel configs on 4.6-rc6, I came across null pointer deference issue in boot stage. CONFIG_SPARSEMEM CONFIG_DEFERRED_STRUCT_PAGE_INIT CONFIG_DEBUG_PAGEALLOC CONFIG_PAGE_EXTENSION CONFIG_DEBUG_VM The splat is: BUG: unable to handle kernel NULL pointer dereference at (null) IP: [<ffffffff8118934b>] page_is_buddy+0x7b/0xe0 PGD 0 Oops: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC Modules linked in: CPU: 3 PID: 106 Comm: pgdatinit1 Not tainted 4.6.0-rc6 #8 Hardware name: Intel Corporation S5520HC/S5520HC, BIOS S5500.86B.01.10.0025.030220091519 03/02/2009 task: ffff88017c1d0040 ti: ffff88017c1d4000 task.ti: ffff88017c1d4000 RIP: 0010:[<ffffffff8118934b>] [<ffffffff8118934b>] page_is_buddy+0x7b/0xe0 RSP: 0000:ffff88017c1d7bf0 EFLAGS: 00010046 RAX: 0000000000000000 RBX: ffffea0019810040 RCX: 0000000000000000 RDX: 0000000000000000 RSI: ffffea0019810040 RDI: 0000000000660401 RBP: ffff88017c1d7c08 R08: 0000000000000001 R09: 0000000000000000 R10: 00000000000001af R11: 0000000000000001 R12: ffffea0019810000 R13: 0000000000000000 R14: 0000000000000009 R15: ffffea0019810000 FS: 0000000000000000(0000) GS:ffff88066cc40000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 0000000000000000 CR3: 0000000002406000 CR4: 00000000000006e0 Stack: 0000000019810000 0000000000000000 ffff88066cfe6080 ffff88017c1d7c70 ffffffff8118bfea 0000000a00000000 0000160000000000 0000000000000001 0000000000000401 ffffea0019810040 0000000000000400 ffff88066cc5aca8 Call Trace: [<ffffffff8118bfea>] __free_one_page+0x23a/0x450 [<ffffffff8118c586>] free_pcppages_bulk+0x136/0x360 [<ffffffff8118cae8>] free_hot_cold_page+0x168/0x1b0 [<ffffffff8118cd8c>] __free_pages+0x5c/0x90 [<ffffffff8260fbf2>] __free_pages_boot_core.isra.70+0x11a/0x14d [<ffffffff8260ff09>] deferred_free_range+0x50/0x62 [<ffffffff8261013b>] deferred_init_memmap+0x220/0x3c3 [<ffffffff8260ff1b>] ? deferred_free_range+0x62/0x62 [<ffffffff8108afc8>] kthread+0xf8/0x110 [<ffffffff81c026b2>] ret_from_fork+0x22/0x40 [<ffffffff8108aed0>] ? kthread_create_on_node+0x200/0x200 Code: 75 7b 48 89 d8 8b 40 1c 85 c0 74 50 48 c7 c6 38 bd 0d 82 48 89 df e8 25 e2 02 00 0f 0b 48 89 f7 89 55 ec e8 18 cb 07 00 8b 55 ec <48> 8b 00 a8 02 74 9d 3b 53 30 75 98 49 8b 14 24 48 8b 03 48 c1 RIP [<ffffffff8118934b>] page_is_buddy+0x7b/0xe0 RSP <ffff88017c1d7bf0> CR2: 0000000000000000 ---[ end trace e0c05a86b43d97f9 ]--- note: pgdatinit1[106] exited with preempt_count 1 I changed page_is_buddy and __free_one_page to non-inline to get more accurate stack trace. Then I did some investigation on it with printing the address of page and buddy, please see the below log: @@@@@@__free_one_page:715: page is at ffffea0005f05c00 buddy is at ffffea0005f05c80, order is 1 @@@@@@__free_one_page:715: page is at ffffea0005f05c00 buddy is at ffffea0005f05d00, order is 2 @@@@@@__free_one_page:715: page is at ffffea0005f05c00 buddy is at ffffea0005f05e00, order is 3 @@@@@@__free_one_page:715: page is at ffffea0019810000 buddy is at ffffea0019810040, order is 0 call trace splat @@@@@@__free_one_page:715: page is at ffffea0005f05bc0 buddy is at ffffea0005f05b80, order is 0 @@@@@@__free_one_page:715: page is at ffffea0005f05b80 buddy is at ffffea0005f05bc0, order is 0 @@@@@@__free_one_page:715: page is at ffffea0005f05b80 buddy is at ffffea0005f05b00, order is 1 @@@@@@__free_one_page:715: page is at ffffea0005f05b40 buddy is at ffffea0005f05b00, order is 0 @@@@@@__free_one_page:715: page is at ffffea0005f05b00 buddy is at ffffea0005f05b40, order is 0 It shows just before the call trace splat, the page address jumped to ffffea0019810000 from ffffea0005f05xxx, not sure why this is happening. Any hint is appreciated.
I think the page address didn't jump, it was that the previous call of __free_one_page() finished at order=3 and now another one was invoked that started at order=0 page ffffea0019810000 and immediately hit the splat.
And, reading the code leads me to the below call path: page_is_buddy() --> page_is_guard() --> lookup_page_ext()
From a quick decodecode I guess that's indeed the path that's crashed. You could try verifying with e.g. addr2line on your vmlinux?
Then lookup_page_ext() just returns null due to the below code: #if defined(CONFIG_DEBUG_VM) || defined(CONFIG_PAGE_POISONING) /* * The sanity checks the page allocator does upon freeing a * page can 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. * * This check is also necessary for ensuring page poisoning * works as expected when enabled */ if (!section->page_ext) return NULL; #endif So, according to the comment, it looks there should be a WARN or BUG if it returns NULL?
The comment reads like it's unlikely, but possible, so WARN/BUG doesn't sounds right.
And, almost no codes check if the return pointer is null or not after lookup_page_ext() is called.
Yep, the callers related to DEBUG_VM (includes page_is_guard()) and page poisoning should probably check the return value. I hope Joonsoo knows something more here :)
Thanks, Yang -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx"> email@xxxxxxxxx </a>
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