linux-kernel - [PATCH] mm: page_is_guard return false when page

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Message-Id: <1463610225-29060-1-git-send-email-yang.shi@linaro.org>
Date:	Wed, 18 May 2016 15:23:45 -0700
From:	Yang Shi <yang.shi@...aro.org>
To:	akpm@...ux-foundation.org, iamjoonsoo.kim@....com
Cc:	vbabka@...e.cz, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linaro-kernel@...ts.linaro.org, yang.shi@...aro.org
Subject: [PATCH] mm: page_is_guard return false when page_ext arrays are not allocated yet

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.

When it returns NULL, page_is_guard() should just return false instead of
checking PAGE_EXT_DEBUG_GUARD unconditionally.

Signed-off-by: Yang Shi <yang.shi@...aro.org>
---
 include/linux/mm.h | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 727f7997..b1f2ad1 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2409,6 +2409,9 @@ static inline bool page_is_guard(struct page *page)
 		return false;
 
 	page_ext = lookup_page_ext(page);
+	if (unlikely(!page_ext))
+		return false;
+
 	return test_bit(PAGE_EXT_DEBUG_GUARD, &page_ext->flags);
 }
 #else
-- 
2.0.2