Re: [PATCH v3 03/14] mm: use pmd lock instead of racy checks in zap_pmd_range()

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Tue, Feb 07, 2017 at 09:11:05AM -0600, Zi Yan wrote:
> >> This causes memory leak or kernel crashing, if VM_BUG_ON() is enabled.
> > 
> > The problem is that numabalancing calls change_huge_pmd() under
> > down_read(mmap_sem), not down_write(mmap_sem) as the rest of users do.
> > It makes numabalancing the only code path beyond page fault that can turn
> > pmd_none() into pmd_trans_huge() under down_read(mmap_sem).
> > 
> > This can lead to race when MADV_DONTNEED miss THP. That's not critical for
> > pagefault vs. MADV_DONTNEED race as we will end up with clear page in that
> > case. Not so much for change_huge_pmd().
> > 
> > Looks like we need pmdp_modify() or something to modify protection bits
> > inplace, without clearing pmd.
> > 
> > Not sure how to get crash scenario.
> > 
> > BTW, Zi, have you observed the crash? Or is it based on code inspection?
> > Any backtraces?
> 
> The problem should be very rare in the upstream kernel. I discover the
> problem in my customized kernel which does very frequent page migration
> and uses numa_protnone.
> 
> The crash scenario I guess is like:
> 1. A huge page pmd entry is in the middle of being changed into either a
> pmd_protnone or a pmd_migration_entry. It is cleared to pmd_none.
> 
> 2. At the same time, the application frees the vma this page belongs to.

Em... no.

This shouldn't be possible: your 1. must be done under down_read(mmap_sem).
And we only be able to remove vma under down_write(mmap_sem), so the
scenario should be excluded.

What do I miss?

> 3. zap_pmd_range() only see pmd_none in
> "if (pmd_trans_huge(*pmd) || pmd_devmap(*pmd))",
> it might catch pmd_protnone in
> "if (pmd_none_or_trans_huge_or_clear_bad(pmd))". But nothing is done for
> it. So the deposited PTE page table page associated with the huge pmd
> entry is not withdrawn.
> 
> 4. free_pmd_range() calls pmd_free_tlb() and in pgtable_pmd_page_dtor(),
> VM_BUG_ON_PAGE(page->pmd_huge_pte, page) is triggered.
> 
> The crash log (you will see a pmd_migration_entry is regarded as bad
> pmd, which should not be. I also saw pmd_protnone before.):
> 
> [ 1945.978677] mm/pgtable-generic.c:33: bad pmd
> ffff8f07b13c1b90(0000004fed803c00)
>                  ^^^^^^^^^^^^^^^^ a pmd migration entry
> 
> [ 1946.964974] page:fffffd1dd0c4f040 count:1 mapcount:-511 mapping:
>      (null) index:0x0
> [ 1946.974265] flags: 0x6ffff0000000000()
> [ 1946.978486] raw: 06ffff0000000000 0000000000000000 0000000000000000
> 00000001fffffe00
> [ 1946.987202] raw: dead000000000100 fffffd1dd0c45c80 ffff8f07aa38e340
> ffff8efdca466678
> [ 1946.995927] page dumped because: VM_BUG_ON_PAGE(page->pmd_huge_pte)
> [ 1947.002984] page->mem_cgroup:ffff8efdca466678
> [ 1947.007927] ------------[ cut here ]------------
> [ 1947.013123] kernel BUG at ./include/linux/mm.h:1733!
> [ 1947.018706] invalid opcode: 0000 [#1] SMP DEBUG_PAGEALLOC
> [ 1947.024774] Modules linked in: ipt_MASQUERADE nf_nat_masquerade_ipv4
> iptable_nat nf_nat_ipv4 nf_nat nf_conntrack_ipv4 nf_defrag_ipv4
> xt_conntrack nf_conntrack intel_rapl sb_edac edac_corei
> [ 1947.077814] CPU: 19 PID: 3303 Comm: python Not tainted
> 4.10.0-rc5-page-migration+ #283
> [ 1947.086721] Hardware name: Dell Inc. PowerEdge R530/0HFG24, BIOS
> 1.5.4 10/05/2015
> [ 1947.095140] task: ffff8f07a5870040 task.stack: ffffc37d64adc000
> [ 1947.101796] RIP: 0010:___pmd_free_tlb+0x83/0x90
> [ 1947.106890] RSP: 0018:ffffc37d64adfce8 EFLAGS: 00010282
> [ 1947.112762] RAX: 0000000000000021 RBX: ffffc37d64adfe10 RCX:
> 0000000000000000
> [ 1947.120770] RDX: 0000000000000000 RSI: ffff8f07c224dea8 RDI:
> ffff8f07c224dea8
> [ 1947.128809] RBP: ffffc37d64adfcf8 R08: 0000000000000001 R09:
> 0000000000000000
> [ 1947.136818] R10: 000000000000000f R11: 0000000000000001 R12:
> fffffd1dd0c4f040
> [ 1947.144825] R13: 00007fae2d7fd000 R14: ffff8f07b13c1b60 R15:
> ffffc37d64adfe10
> [ 1947.152832] FS:  00007fafbcce6700(0000) GS:ffff8f07c2240000(0000)
> knlGS:0000000000000000
> [ 1947.161934] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [ 1947.168380] CR2: 00007fafeb851188 CR3: 0000001432184000 CR4:
> 00000000001406e0
> [ 1947.176393] Call Trace:
> [ 1947.179160]  free_pgd_range+0x487/0x5d0
> [ 1947.183476]  free_pgtables+0xc4/0x120
> [ 1947.187593]  unmap_region+0xe1/0x130
> [ 1947.191620]  do_munmap+0x273/0x400
> [ 1947.195452]  SyS_munmap+0x53/0x70
> [ 1947.199190]  entry_SYSCALL_64_fastpath+0x23/0xc6
> [ 1947.204382] RIP: 0033:0x7fafeb59d387
> [ 1947.208406] RSP: 002b:00007fafbcce5358 EFLAGS: 00000207 ORIG_RAX:
> 000000000000000b
> [ 1947.216924] RAX: ffffffffffffffda RBX: 00007faf2c0b6780 RCX:
> 00007fafeb59d387
> [ 1947.224933] RDX: 00007fae247fc030 RSI: 0000000009001000 RDI:
> 00007fae247fc000
> [ 1947.232940] RBP: 00007fafbcce5390 R08: 00007faf48f9ed00 R09:
> 0000000000000100
> [ 1947.240947] R10: 0000000000000020 R11: 0000000000000207 R12:
> 0000000002cf1fc0
> [ 1947.248955] R13: 0000000002cf1fc0 R14: 000000000343d530 R15:
> 00007fafbcce5810
> [ 1947.256965] Code: 4c 89 e6 48 89 df e8 0d b5 1a 00 84 c0 74 08 48 89
> df e8 91 b4 1a 00 5b 41 5c 5d c3 48 c7 c6 d8 73 c7 b8 4c 89 e7 e8 dd 7b
> 1a 00 <0f> 0b 48 8b 3d 34 80 d9 00 eb 99 66 90
> [ 1947.278200] RIP: ___pmd_free_tlb+0x83/0x90 RSP: ffffc37d64adfce8
> [ 1947.285688] ---[ end trace 7864a23976d71e0a ]---
> 
> -- 
> Best Regards,
> Yan Zi
> 



-- 
 Kirill A. Shutemov

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