linux-kernel - Re: arm/ksm: Unable to handle kernel paging request in get_ksm_page() and ksm_scan

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Date:	Mon, 30 Mar 2015 11:06:47 +0800
From:	Xishi Qiu <qiuxishi@...wei.com>
To:	Hugh Dickins <hughd@...gle.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>, <neilb@...e.de>,
	<heiko.carstens@...ibm.com>, <dhowells@...hat.com>,
	<izik.eidus@...ellosystems.com>, <aarcange@...hat.com>,
	<chrisw@...s-sol.org>, Linux MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>, <weiyuan.wei@...wei.com>
Subject: Re: arm/ksm: Unable to handle kernel paging request in get_ksm_page()
 and ksm_scan_thread()

On 2015/3/30 9:46, Xishi Qiu wrote:

> On 2015/3/30 8:43, Hugh Dickins wrote:
> 
>> On Sat, 28 Mar 2015, Xishi Qiu wrote:
>>> On 2015/3/26 21:23, Xishi Qiu wrote:
>>>
>>>> Here are two panic logs from smart phone test, and the kernel version is v3.10.
>>>>
>>>> log1 is "Unable to handle kernel paging request at virtual address c0704da020", it should be ffffffc0704da020, right?
>>
>> That one was an oops at get_ksm_page+0x34/0x150: I'm pretty sure that
>> comes from the "kpfn = ACCESS_ONCE(stable_node->kpfn)" line, that the
>> stable_node pointer (in x21 or x22) has upper bits cleared; which
>> suggests corruption of the rmap_item supposed to point to it.
>>
>> get_ksm_page() is tricky with ACCESS_ONCEs against page migration,
>> and the structures tricky with unions; but pointers overlay pointers
>> in those unions, I don't see any way we might pick up an address with
>> the upper 24 or 32 bits cleared due to that.
>>
>>>> and log2 is "Unable to handle kernel paging request at virtual address 1e000796", it should be ffffffc01e000796, right?
>>
>> And this one was an oops at ksm_scan_thread+0x4ac/0xce0; as is the oops
>> you posted below.  Which contains lots of hex numbers, but very little
>> info I can work from.
>>
>> Please make a CONFIG_DEBUG_INFO=y build of one of the kernels you're
>> hitting this with, then use the disassembler (objdump -ld perhaps) to
>> identify precisely which line of ksm.c that is oopsing on: the compiler
>> will have inlined more interesting functions into ksm_scan_thread, so
>> I haven't a clue where it's actually oopsing.
>>
>> Maybe we'll find that it's also oopsing on a kernel virtual address
>> from an rmap_item, maybe we won't.
>>
>> And I don't read arm64 assembler at all, so I shall be rather limited
>> in what I can tell you, I'm afraid.
>>
>>>>
>>>> I cann't repeat the panic by test, so could anyone tell me this is the 
>>>> bug of ksm or other reason?
>>
>> I've not heard of any problem like this with KSM on other architectures.
>> Maybe it is making some assumption which is invalid on arm64, but I'd
>> have thought we'd have heard about that before now.  My guess is that
>> something in your kernel is stamping on KSM's structures.
>>
>> A relevant experiment (after identifying the oops line in your current
>> kernel) might be to switch from CONFIG_SLAB=y to CONFIG_SLUB=y or vice
>> versa.  I doubt SLAB or SLUB is to blame, but changing allocator might
>> shake things up in a way that either hides the problem, or shifts it
>> elsewhere.
>>
>> Hugh
>>
> 
> Hi Hugh,
> 
> Thanks for your reply. There are 3 cases as follows, at first I think maybe
> something causes the oops, but all of the cases are relevant to "rmap_item",
> so I have no idea.
> 
> 1. ksm_scan_thread+0xa88/0xce0 -> unstable_tree_search_insert() -> tree_rmap_item = rb_entry(*new, struct rmap_item, node);
> 
> 2. ksm_scan_thread+0x4ac/0xce0 -> get_next_rmap_item() -> if ((rmap_item->address & PAGE_MASK) == addr)
> 
> 3. get_ksm_page+0x34/0x150 -> get_ksm_page() -> kpfn = ACCESS_ONCE(stable_node->kpfn);
> 
> Thanks,
> Xishi Qiu
> 

The kernel is v3.10.59

> 
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