Re: [PATCH RFC] mm: Fix kernel BUG when userfaultfd_move encounters swapcache

[Peter Xu <peterx@redhat.com>]

On Thu, Feb 20, 2025 at 12:25:19AM +1300, Barry Song wrote:
> @@ -1079,9 +1080,19 @@ static int move_swap_pte(struct mm_struct *mm,
>  			 pmd_t *dst_pmd, pmd_t dst_pmdval,
>  			 spinlock_t *dst_ptl, spinlock_t *src_ptl)
>  {
> +	struct folio *folio;
> +	swp_entry_t entry;
> +
>  	if (!pte_swp_exclusive(orig_src_pte))
>  		return -EBUSY;
>  
> +	entry = pte_to_swp_entry(orig_src_pte);
> +	folio = filemap_get_folio(swap_address_space(entry), swap_cache_index(entry));

[Besides what's being discussed elsewhere..]

swap_cache_get_folio() says:

 * Caller must lock the swap device or hold a reference to keep it valid.

Do we need get_swap_device() too here to avoid swapoff race?

> +	if (!IS_ERR(folio)) {
> +		folio_put(folio);
> +		return -EBUSY;
> +	}
> +
>  	double_pt_lock(dst_ptl, src_ptl);
>  
>  	if (!is_pte_pages_stable(dst_pte, src_pte, orig_dst_pte, orig_src_pte,
> -- 
> 2.39.3 (Apple Git-146)
> 

-- 
Peter Xu