a racy access flag clearing warning when calling mmap system call

[chenjiankang]

Hi will
    
   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
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