Re: [PATCH -mm] mm, gup: prevent pmd checking race in follow_pmd_mask()

[huang ying <huang.ying.caritas@gmail.com>]

On Wed, Apr 4, 2018 at 11:02 PM, Zi Yan <zi.yan@cs.rutgers.edu> wrote:
> On 3 Apr 2018, at 23:22, Huang, Ying wrote:
>
>> From: Huang Ying <ying.huang@intel.com>
>>
>> mmap_sem will be read locked when calling follow_pmd_mask().  But this
>> cannot prevent PMD from being changed for all cases when PTL is
>> unlocked, for example, from pmd_trans_huge() to pmd_none() via
>> MADV_DONTNEED.  So it is possible for the pmd_present() check in
>> follow_pmd_mask() encounter a none PMD.  This may cause incorrect
>> VM_BUG_ON() or infinite loop.  Fixed this via reading PMD entry again
>> but only once and checking the local variable and pmd_none() in the
>> retry loop.
>>
>> As Kirill pointed out, with PTL unlocked, the *pmd may be changed
>> under us, so read it directly again and again may incur weird bugs.
>> So although using *pmd directly other than pmd_present() checking may
>> be safe, it is still better to replace them to read *pmd once and
>> check the local variable for multiple times.
>
> I see you point there. The patch wants to provide a consistent value
> for all race checks. Specifically, this patch is trying to avoid the inconsistent
> reads of *pmd for if-statements, which causes problem when both if-condition reads *pmd and
> the statements inside "if" reads *pmd again and two reads can give different values.
> Am I right about this?

Yes.

> If yes, the problem can be solved by something like:
>
> if (!pmd_present(tmpval = *pmd)) {
>     check tmpval instead of *pmd;
> }
>
> Right?

I think this isn't enough yet.  we need

tmpval = READ_ONCE(*pmd);

To prevent compiler to generate code to read *pmd again and again.
Please check the comments of pmd_none_or_trans_huge_or_clear_bad()
about barrier.

Best Regards,
Huang, Ying

> I just wonder if we need some general code for all race checks.
>
> Thanks.
>
> --
> Best Regards
> Yan Zi