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

[Xishi Qiu]

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


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