linux-kernel - Re: [PATCH] userfaultfd: change src_folio after ensuring it's unpinned in UFFDIO

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Date: Thu, 4 Apr 2024 18:21:50 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Lokesh Gidra <lokeshgidra@...gle.com>
Cc: akpm@...ux-foundation.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, surenb@...gle.com,
	kernel-team@...roid.com, aarcange@...hat.com, peterx@...hat.com,
	david@...hat.com, zhengqi.arch@...edance.com,
	kaleshsingh@...gle.com, ngeoffray@...gle.com
Subject: Re: [PATCH] userfaultfd: change src_folio after ensuring it's
 unpinned in UFFDIO_MOVE

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?