Re: [PATCH] mm: page_is_guard return false when page_ext arrays are not allocated yet

[Joonsoo Kim]

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().

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?

And, above comment would be stale because it comes from when memcg uses
this struct page extension funtionality. Now, memcg doesn't use it and
there are some changes on this area so I'm not sure that is still true.

Thanks.