Re: [PATCH 03/11] mm: page_vma_mapped_walk(): use pmd_read_atomic()

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Wed, Jun 09, 2021 at 11:38:11PM -0700, Hugh Dickins wrote:
> page_vma_mapped_walk() cleanup: use pmd_read_atomic() with barrier()
> instead of READ_ONCE() for pmde: some architectures (e.g. i386 with PAE)
> have a multi-word pmd entry, for which READ_ONCE() is not good enough.
> 
> Signed-off-by: Hugh Dickins <hughd@google.com>
> Cc: <stable@vger.kernel.org>
> ---
>  mm/page_vma_mapped.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
> index 7c0504641fb8..973c3c4e72cc 100644
> --- a/mm/page_vma_mapped.c
> +++ b/mm/page_vma_mapped.c
> @@ -182,13 +182,16 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
>  	pud = pud_offset(p4d, pvmw->address);
>  	if (!pud_present(*pud))
>  		return false;
> +
>  	pvmw->pmd = pmd_offset(pud, pvmw->address);
>  	/*
>  	 * Make sure the pmd value isn't cached in a register by the
>  	 * compiler and used as a stale value after we've observed a
>  	 * subsequent update.
>  	 */
> -	pmde = READ_ONCE(*pvmw->pmd);
> +	pmde = pmd_read_atomic(pvmw->pmd);
> +	barrier();
> +

Hm. It makes me wounder if barrier() has to be part of pmd_read_atomic().
mm/hmm.c uses the same pattern as you are and I tend to think that the
rest of pmd_read_atomic() users may be broken.

Am I wrong?

-- 
 Kirill A. Shutemov