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

[Zi Yan]

Kirill A. Shutemov wrote:
> 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?

You are right. This problem will not happen in the upstream kernel.

The problem comes from my customized kernel, where I migrate pages away
instead of reclaiming them when memory is under pressure. I did not take
any mmap_sem when I migrate pages. So I got this error.

It is a false alarm. Sorry about that. Thanks for clarifying the problem.


-- 
Best Regards,
Yan Zi

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