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

[Shi, Yang]

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".

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.

Thanks,
Yang

> 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.