Re: [PATCH mm-unstable v2] mm/hugetlb_vmemmap: fix race with speculative PFN walkers

[Andrew Morton]

On Thu, 27 Jun 2024 16:27:05 -0600 Yu Zhao <yuzhao@xxxxxxxxxx> wrote:

> While investigating HVO for THPs [1], it turns out that speculative
> PFN walkers like compaction can race with vmemmap modifications, e.g.,
> 
>   CPU 1 (vmemmap modifier)         CPU 2 (speculative PFN walker)
>   -------------------------------  ------------------------------
>   Allocates an LRU folio page1
>                                    Sees page1
>   Frees page1
> 
>   Allocates a hugeTLB folio page2
>   (page1 being a tail of page2)
> 
>   Updates vmemmap mapping page1
>                                    get_page_unless_zero(page1)
> 
> Even though page1->_refcount is zero after HVO, get_page_unless_zero()
> can still try to modify this read-only field, resulting in a crash.

Ah.  So we should backport this into earlier kernels, yes?

Are we able to identify a Fixes: for this?  Looks difficult.

This seems quite hard to trigger.  Do any particular userspace actions
invoke the race?