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

[Zi Yan]

On 3 Apr 2018, at 23:22, Huang, Ying wrote:

> From: Huang Ying <ying.huang@xxxxxxxxx>
>
> 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?

If yes, the problem can be solved by something like:

if (!pmd_present(tmpval = *pmd)) {
    check tmpval instead of *pmd;
}

Right?

I just wonder if we need some general code for all race checks.

Thanks.

--
Best Regards
Yan Zi

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