[v2 PATCH 1/2] mm: gup: fix the fast GUP race against THP collapse

[Andrew Morton]

On Wed,  7 Sep 2022 11:01:43 -0700 Yang Shi <shy828301 at gmail.com> wrote:

> Since general RCU GUP fast was introduced in commit 2667f50e8b81 ("mm:
> introduce a general RCU get_user_pages_fast()"), a TLB flush is no longer
> sufficient to handle concurrent GUP-fast in all cases, it only handles
> traditional IPI-based GUP-fast correctly.  On architectures that send
> an IPI broadcast on TLB flush, it works as expected.  But on the
> architectures that do not use IPI to broadcast TLB flush, it may have
> the below race:
> 
>    CPU A                                          CPU B
> THP collapse                                     fast GUP
>                                               gup_pmd_range() <-- see valid pmd
>                                                   gup_pte_range() <-- work on pte
> pmdp_collapse_flush() <-- clear pmd and flush
> __collapse_huge_page_isolate()
>     check page pinned <-- before GUP bump refcount
>                                                       pin the page
>                                                       check PTE <-- no change
> __collapse_huge_page_copy()
>     copy data to huge page
>     ptep_clear()
> install huge pmd for the huge page
>                                                       return the stale page
> discard the stale page
> 
> The race could be fixed by checking whether PMD is changed or not after
> taking the page pin in fast GUP, just like what it does for PTE.  If the
> PMD is changed it means there may be parallel THP collapse, so GUP
> should back off.
> 
> Also update the stale comment about serializing against fast GUP in
> khugepaged.
> 
> Fixes: 2667f50e8b81 ("mm: introduce a general RCU get_user_pages_fast()")

Is this not worth a -stable backport?