Re: [PATCH] userfaultfd: change src_folio after ensuring it's unpinned in UFFDIO_MOVE

[Matthew Wilcox]

On Thu, Apr 04, 2024 at 10:17:26AM -0700, Lokesh Gidra wrote:
> -		folio_move_anon_rmap(src_folio, dst_vma);
> -		WRITE_ONCE(src_folio->index, linear_page_index(dst_vma, dst_addr));
> -
>  		src_pmdval = pmdp_huge_clear_flush(src_vma, src_addr, src_pmd);
>  		/* Folio got pinned from under us. Put it back and fail the move. */
>  		if (folio_maybe_dma_pinned(src_folio)) {
> @@ -2270,6 +2267,9 @@ int move_pages_huge_pmd(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd, pm
>  			goto unlock_ptls;
>  		}
>  
> +		folio_move_anon_rmap(src_folio, dst_vma);
> +		WRITE_ONCE(src_folio->index, linear_page_index(dst_vma, dst_addr));
> +

This use of WRITE_ONCE scares me.  We hold the folio locked.  Why do
we need to use WRITE_ONCE?  Who's looking at folio->index without
holding the folio lock?