Re: [PATCH] mm/rmap: Fix potential pte_unmap on an not mapped pte

[Andrew Morton]

On Wed, 27 Jan 2021 04:33:49 -0500 Miaohe Lin <linmiaohe@xxxxxxxxxx> wrote:

> For PMD-mapped page (usually THP), pvmw->pte is NULL. For PTE-mapped THP,
> pvmw->pte is mapped. But for HugeTLB pages, pvmw->pte is not mapped and set
> to the relevant page table entry. So in page_vma_mapped_walk_done(), we may
> do pte_unmap() for HugeTLB pte which is not mapped. Fix this by checking
> pvmw->page against PageHuge before trying to do pte_unmap().
> 

What are the runtime consequences of this?  Is there a workload which
is known to trigger it?

IOW, how do we justify a -stable backport of this fix?

>
> --- a/include/linux/rmap.h
> +++ b/include/linux/rmap.h
> @@ -213,7 +213,8 @@ struct page_vma_mapped_walk {
>  
>  static inline void page_vma_mapped_walk_done(struct page_vma_mapped_walk *pvmw)
>  {
> -	if (pvmw->pte)
> +	/* HugeTLB pte is set to the relevant page table entry without pte_mapped. */
> +	if (pvmw->pte && !PageHuge(pvmw->page))
>  		pte_unmap(pvmw->pte);
>  	if (pvmw->ptl)
>  		spin_unlock(pvmw->ptl);
> -- 
> 2.19.1