Re: a racy access flag clearing warning when calling mmap system call

[chenjiankang]

在 2017/12/1 21:18, Will Deacon 写道:
> On Fri, Dec 01, 2017 at 03:38:04PM +0800, chenjiankang wrote:
>>    I find a warning by a syzkaller test;
>>
>>    When the mmap syscall is called to create a virtual memory,
>> firstly it delete a old huge page mapping area;   
>>    Before splitting the huge page, the pmd of a huge page is set up.
>> But The PTE_AF is zreo belonging to the current pmd of huge page.
>>    I suspect that when the child process is created, the PTE_AF is cleared in copy_one_pte();
>> So, the set_pte_at() can have a warning.
>>    
>>    There, whether the set_pte_at() should detect PTE_AF?
>>
>> Thanks
>> The log:
>>
>>     set_pte_at: racy access flag clearing: 00e000009d200bd1 -> 01e000009d200bd1
>> ------------[ cut here ]------------      
>> WARNING: at ../../../../../kernel/linux-4.1/arch/arm64/include/asm/pgtable.h:211
> 
> Given that this is a fairly old 4.1 kernel, could you try to reproduce the
> failure with something more recent, please? We've fixed many bugs since
> then, some of them involving huge pages.
> 
> Thanks,
> 
> Will

 Hi Will:
   
    It is more difficult to reproduce the failure with the old 4.1 kernel.
Does the warning have an impact? Or this is just to lose a PTE_AF flag?

 Thanks 
  
 Jiankang 

> 
>> Modules linked in:                         
>> CPU: 0 PID: 3665 Comm: syz-executor7 Not tainted 4.1.44+ #1
>> Hardware name: linux,dummy-virt (DT)       
>> task: ffffffc06a873fc0 ti: ffffffc05aefc000 task.ti: ffffffc05aefc000
>> PC is at pmdp_splitting_flush+0x194/0x1b0
>> LR is at pmdp_splitting_flush+0x194/0x1b0
>> pc : [<ffffffc0000a5244>] lr : [<ffffffc0000a5244>] pstate: 80000145
>> sp : ffffffc05aeff770                     
>> x29: ffffffc05aeff770 x28: ffffffc05ae45800 
>> x27: 0000000020200000 x26: ffffffc061fdf450 
>> x25: 0000000000020000 x24: 0000000000000001 
>> x23: ffffffc06333d9c8 x22: ffffffc0014ba000 
>> x21: 01e000009d200bd1 x20: 00e000009d200bd1 
>> x19: ffffffc05ae45800 x18: 0000000000000000 
>> x17: 00000000004b4000 x16: ffffffc00017fdc0 
>> x15: 00000000038ee280 x14: 3030653130203e2d 
>> x13: 2031646230303264 x12: 3930303030306530 
>> x11: 30203a676e697261 x10: 656c632067616c66 
>> x9 : 2073736563636120 x8 : 79636172203a7461 
>> x7 : ffffffc05aeff430 x6 : ffffffc00015f38c 
>> x5 : 0000000000000003 x4 : 0000000000000000 
>> x3 : 0000000000000003 x2 : 0000000000010000 
>> x1 : ffffff9005a03000 x0 : 000000000000004b 
>> Call trace:                                
>> [<ffffffc0000a5244>] pmdp_splitting_flush+0x194/0x1b0
>> [<ffffffc0002784c0>] split_huge_page_to_list+0x168/0xdb0
>> [<ffffffc00027a0d8>] __split_huge_page_pmd+0x1b0/0x510
>> [<ffffffc00027b5ac>] split_huge_page_pmd_mm+0x84/0x88
>> [<ffffffc00027b674>] split_huge_page_address+0xc4/0xe8
>> [<ffffffc00027b7f4>] __vma_adjust_trans_huge+0x15c/0x190
>> [<ffffffc000238d5c>] vma_adjust+0x884/0x9f0
>> [<ffffffc0002390c8>] __split_vma.isra.5+0x200/0x310
>> [<ffffffc00023b7b8>] do_munmap+0x5e0/0x608 
>> [<ffffffc00023c0d4>] mmap_region+0x12c/0x900
>> [<ffffffc00023cd2c>] do_mmap_pgoff+0x484/0x540
>> [<ffffffc000218118>] vm_mmap_pgoff+0x128/0x158
>> [<ffffffc000239920>] SyS_mmap_pgoff+0x188/0x300
>> [<ffffffc00008cce0>] sys_mmap+0x58/0x80   
>>
> 
> .
>