Re: [PATCH 2/2] mm: Fix a huge pud insertion race during faulting

[Andrew Morton]

On Fri, 15 Nov 2019 12:58:08 +0100 Thomas Hellström (VMware) <thomas_os@xxxxxxxxxxxx> wrote:

> A huge pud page can theoretically be faulted in racing with pmd_alloc()
> in __handle_mm_fault(). That will lead to pmd_alloc() returning an
> invalid pmd pointer. Fix this by adding a pud_trans_unstable() function
> similar to pmd_trans_unstable() and check whether the pud is really stable
> before using the pmd pointer.
> 
> Race:
> Thread 1:             Thread 2:                 Comment
> create_huge_pud()                               Fallback - not taken.
> 		      create_huge_pud()         Taken.
> pmd_alloc()                                     Returns an invalid pointer.

What are the user-visible runtime effects of this change?

Is a -stable backport warranted?