Re: [PATCH v1] mm/madvise: Always set ptes via arch helpers

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Fri, Mar 07, 2025 at 12:33:06PM +0000, Ryan Roberts wrote:
> Instead of writing a pte directly into the table, use the set_pte_at()
> helper, which gives the arch visibility of the change.
>
> In this instance we are guaranteed that the pte was originally none and
> is being modified to a not-present pte, so there was unlikely to be a
> bug in practice (at least not on arm64). But it's bad practice to write
> the page table memory directly without arch involvement.
>
> Cc: <stable@vger.kernel.org>
> Fixes: 662df3e5c376 ("mm: madvise: implement lightweight guard page mechanism")
> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
> ---
>  mm/madvise.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/madvise.c b/mm/madvise.c
> index 388dc289b5d1..6170f4acc14f 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -1101,7 +1101,7 @@ static int guard_install_set_pte(unsigned long addr, unsigned long next,
>  	unsigned long *nr_pages = (unsigned long *)walk->private;
>
>  	/* Simply install a PTE marker, this causes segfault on access. */
> -	*ptep = make_pte_marker(PTE_MARKER_GUARD);
> +	set_pte_at(walk->mm, addr, ptep, make_pte_marker(PTE_MARKER_GUARD));

I agree with you, but I think perhaps the arg name here is misleading :) If
you look at mm/pagewalk.c and specifically, in walk_pte_range_inner():

		if (ops->install_pte && pte_none(ptep_get(pte))) {
			pte_t new_pte;

			err = ops->install_pte(addr, addr + PAGE_SIZE, &new_pte,
					       walk);
			if (err)
				break;

			set_pte_at(walk->mm, addr, pte, new_pte);

			...
		}

So the ptep being assigned here is a stack value, new_pte, which we simply
assign to, and _then_ the page walker code handles the set_pte_at() for us.

So we are indeed doing the right thing here, just in a different place :P

>  	(*nr_pages)++;
>
>  	return 0;
> --
> 2.43.0
>