Re: [PATCH V2] mm: fix kernel crash in khugepaged thread

From: Vlastimil Babka
Date: Thu Nov 05 2015 - 03:12:42 EST

On 10/29/2015 01:35 AM, Kirill A. Shutemov wrote:
> On Mon, Oct 26, 2015 at 06:39:20PM +0800, yalin wang wrote:
>> This crash is caused by NULL pointer deference, in page_to_pfn() marco,
>> when page == NULL :
>>
>> [ 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 | 16 +++++++---------
>> 1 file changed, 7 insertions(+), 9 deletions(-)
>>
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 4b3420a..8a3482f 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -2492,7 +2492,7 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
>> struct page **hpage)
>> {
>> pmd_t *pmd;
>> - pte_t *pte, *_pte;
>> + pte_t *pte, *_pte, uninitialized_var(pteval);
>
> Oh.. That's ugly.
>
> Can't we restructure the code to get rid of this?
>
> https://lwn.net/Articles/529954/
>
>> int ret = 0, none_or_zero = 0, result = 0;
>> struct page *page = NULL;
>> unsigned long _address;
>> @@ -2503,16 +2503,14 @@ 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 ret;
>>
>> memset(khugepaged_node_load, 0, sizeof(khugepaged_node_load));
>> pte = pte_offset_map_lock(mm, pmd, address, &ptl);
>> for (_address = address, _pte = pte; _pte < pte+HPAGE_PMD_NR;
>> _pte++, _address += PAGE_SIZE) {
>> - pte_t pteval = *_pte;
>> + pteval = *_pte;
>> if (is_swap_pte(pteval)) {
>> if (++unmapped <= khugepaged_max_ptes_swap) {
>> continue;
>> @@ -2605,9 +2603,9 @@ out_unmap:
>> /* collapse_huge_page will return with the mmap_sem released */
>> 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);
>> + trace_mm_khugepaged_scan_pmd(mm, pte_present(pteval) ?
>> + pte_pfn(pteval) : -1, writable, referenced,
>> + none_or_zero, result, unmapped);
>
> maybe passing down pte instead of pfn?

Maybe just pass the page, and have tracepoint's fast assign check for !NULL and
do page_to_pfn itself? That way the complexity and overhead is only in the
tracepoint and when enabled.

Vlastimil

>> return ret;
>> }
>>
>> --
>> 1.9.1
>>
>> --
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>

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