Re: [PATCH] mm: fix kernel crash in khugepaged thread

[yalin wang]

> On Oct 23, 2015, at 18:11, Kirill A. Shutemov <kirill@xxxxxxxxxxxxx> wrote:
> 
> On Fri, Oct 23, 2015 at 05:38:49PM +0800, yalin wang wrote:
>> This crash is caused by NULL pointer deference:
>> [  182.639154 ] Unable to handle kernel NULL pointer dereference at virtual address 00000000
>> [  182.639491 ] pgd = ffffffc00077a000
>> [  182.639761 ] [00000000] *pgd=00000000b9422003, *pud=00000000b9422003, *pmd=00000000b9423003, *pte=0060000008000707
>> [  182.640749 ] Internal error: Oops: 94000006 [#1] SMP
>> [  182.641197 ] Modules linked in:
>> [  182.641580 ] CPU: 1 PID: 26 Comm: khugepaged Tainted: G        W       4.3.0-rc6-next-20151022ajb-00001-g32f3386-dirty #3
>> [  182.642077 ] Hardware name: linux,dummy-virt (DT)
>> [  182.642227 ] task: ffffffc07957c080 ti: ffffffc079638000 task.ti: ffffffc079638000
>> [  182.642598 ] PC is at khugepaged+0x378/0x1af8
>> [  182.642826 ] LR is at khugepaged+0x418/0x1af8
>> [  182.643047 ] pc : [<ffffffc0001980ac>] lr : [<ffffffc00019814c>] pstate: 60000145
>> [  182.643490 ] sp : ffffffc07963bca0
>> [  182.643650 ] x29: ffffffc07963bca0 x28: ffffffc00075c000
>> [  182.644024 ] x27: ffffffc00f275040 x26: ffffffc0006c7000
>> [  182.644334 ] x25: 00e8000048800f51 x24: 0000000006400000
>> [  182.644687 ] x23: 0000000000000002 x22: 0000000000000000
>> [  182.644972 ] x21: 0000000000000000 x20: 0000000000000000
>> [  182.645446 ] x19: 0000000000000000 x18: 0000007ff86d0990
>> [  182.645931 ] x17: 00000000007ef9c8 x16: ffffffc000098390
>> [  182.646236 ] x15: ffffffffffffffff x14: 00000000ffffffff
>> [  182.646649 ] x13: 000000000000016a x12: 0000000000000000
>> [  182.647046 ] x11: ffffffc07f025020 x10: 0000000000000000
>> [  182.647395 ] x9 : 0000000000000048 x8 : ffffffc000721e28
>> [  182.647872 ] x7 : 0000000000000000 x6 : ffffffc07f02d000
>> [  182.648261 ] x5 : fffffffffffffe00 x4 : ffffffc00f275040
>> [  182.648611 ] x3 : 0000000000000000 x2 : ffffffc00f2ad000
>> [  182.648908 ] x1 : 0000000000000000 x0 : ffffffc000727000
>> [  182.649147 ]
>> [  182.649252 ] Process khugepaged (pid: 26, stack limit = 0xffffffc079638020)
>> [  182.649724 ] Stack: (0xffffffc07963bca0 to 0xffffffc07963c000)
>> [  182.650141 ] bca0: ffffffc07963be30 ffffffc0000b5044 ffffffc07961fb80 ffffffc00072e630
>> [  182.650587 ] bcc0: ffffffc0005d5090 0000000000000000 ffffffc000197d34 0000000000000000
>> [  182.651009 ] bce0: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
>> [  182.651446 ] bd00: ffffffc07963bd90 ffffffc07f1cbf80 000000004f3be003 ffffffc00f2750a4
>> [  182.651956 ] bd20: ffffffc00f3bf000 ffffffc000000001 0000000000000001 ffffffc07f085740
>> [  182.652520 ] bd40: ffffffc00f2ad188 ffffffc000000000 0000000006200000 ffffffc00f275040
>> [  182.652972 ] bd60: ffffffc0006b1a90 ffffffc079638000 ffffffc07963be20 ffffffc00f0144d0
>> [  182.653357 ] bd80: ffffffc000000000 0000000006400000 ffffffc00f0144d0 00000a0800000001
>> [  182.653793 ] bda0: 0000100000000001 ffffffc000000001 ffffffc07f025000 ffffffc00f2750a8
>> [  182.654226 ] bdc0: 00000001000005f8 ffffffc00075a000 0000000006a00000 ffffffc000727000
>> [  182.654522 ] bde0: ffffffc0006e8478 ffffffc000000000 0000000100000000 ffffffc078fb9000
>> [  182.654869 ] be00: ffffffc07963be30 ffffffc000000000 ffffffc07957c080 ffffffc0000cfc4c
>> [  182.655225 ] be20: ffffffc07963be20 ffffffc07963be20 0000000000000000 ffffffc000085c50
>> [  182.655588 ] be40: ffffffc0000b4f64 ffffffc07961fb80 0000000000000000 0000000000000000
>> [  182.656138 ] be60: 0000000000000000 ffffffc0000bee2c ffffffc0000b4f64 0000000000000000
>> [  182.656609 ] be80: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
>> [  182.657145 ] bea0: ffffffc07963bea0 ffffffc07963bea0 0000000000000000 ffffffc000000000
>> [  182.657475 ] bec0: ffffffc07963bec0 ffffffc07963bec0 0000000000000000 0000000000000000
>> [  182.657922 ] bee0: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
>> [  182.658558 ] bf00: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
>> [  182.658972 ] bf20: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
>> [  182.659291 ] bf40: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
>> [  182.659722 ] bf60: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
>> [  182.660122 ] bf80: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
>> [  182.660654 ] bfa0: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
>> [  182.661064 ] bfc0: 0000000000000000 0000000000000000 0000000000000000 0000000000000005
>> [  182.661466 ] bfe0: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
>> [  182.661848 ] Call trace:
>> [  182.662050 ] [<ffffffc0001980ac>] khugepaged+0x378/0x1af8
>> [  182.662294 ] [<ffffffc0000b5040>] kthread+0xdc/0xf4
>> [  182.662605 ] [<ffffffc000085c4c>] ret_from_fork+0xc/0x40
>> [  182.663046 ] Code: 35001700 f0002c60 aa0703e3 f9009fa0 (f94000e0)
>> [  182.663901 ] ---[ end trace 637503d8e28ae69e  ]---
>> [  182.664160 ] Kernel panic - not syncing: Fatal exception
>> [  182.664571 ] CPU2: stopping
>> [  182.664794 ] CPU: 2 PID: 0 Comm: swapper/2 Tainted: G      D W       4.3.0-rc6-next-20151022ajb-00001-g32f3386-dirty #3
>> [  182.665248 ] Hardware name: linux,dummy-virt (DT)
>> 
>> Signed-off-by: yalin wang <yalin.wang2010@xxxxxxxxx>
>> ---
>> mm/huge_memory.c | 5 +++--
>> 1 file changed, 3 insertions(+), 2 deletions(-)
>> 
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 4b3420a..a5f4d9c 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -2606,8 +2606,9 @@ out_unmap:
>> 		collapse_huge_page(mm, address, hpage, vma, node);
>> 	}
>> out:
>> -	trace_mm_khugepaged_scan_pmd(mm, page_to_pfn(page), writable, referenced,
>> -				     none_or_zero, result, unmapped);
>> +	if (page)
>> +		trace_mm_khugepaged_scan_pmd(mm, page_to_pfn(page), writable,
>> +				referenced, none_or_zero, result, unmapped);
>> 	return ret;
>> }
> 
> What about this instead?
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 4b3420ade697..392ebba27fe2 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2503,10 +2503,8 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
> 	VM_BUG_ON(address & ~HPAGE_PMD_MASK);
> 
> 	pmd = mm_find_pmd(mm, address);
> -	if (!pmd) {
> -		result = SCAN_PMD_NULL;
> -		goto out;
> -	}
> +	if (!pmd)
> +		return 0;

it is not safe to add return here,
there is lots of place which goto out_unmap below mm_find_pmd() ,
like  page = vm_normal_page(vma, _address, pteval);
if page == NULL here , will also result in NULL pointer deference crash .
Thanks


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