Re: [PATCH 2/2] mm/madvise: Use set_pte() to write page tables

[Lorenzo Stoakes]

On Thu, Dec 11, 2025 at 09:43:58AM +0000, Ryan Roberts wrote:
> On 11/12/2025 08:11, Samuel Holland wrote:
> > Generic code must always use the architecture-provided helper function
> > to write page tables.
> >
> > Fixes: 662df3e5c376 ("mm: madvise: implement lightweight guard page mechanism")
> > Signed-off-by: Samuel Holland <samuel.holland@xxxxxxxxxx>
> > ---
> >
> >  mm/madvise.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/mm/madvise.c b/mm/madvise.c
> > index b617b1be0f535..4da9c32f8738a 100644
> > --- a/mm/madvise.c
> > +++ b/mm/madvise.c
> > @@ -1114,7 +1114,7 @@ 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(ptep, make_pte_marker(PTE_MARKER_GUARD));
>
> No! As I explained in my response on the other thread (which you linked in the
> cover letter), it is correct as is and should not be changed to set_pte().

Yup agreed, esp. given this is my code :)

Also some arches don't define set_pte()... it seems set_xxx() functions not
really intended to be used outside of arch code - see
e.g. https://elixir.bootlin.com/linux/v6.18.1/A/ident/set_pte

>
> Copy/pasting my explanation:
>
> | I tried "fixing" this before. But it's correct as is. ptep is pointing to a
> | value on the stack. See [2].
> |
> | https://lore.kernel.org/linux-mm/2308a4d0-273e-4cf8-9c9f-3008c42b6d18@xxxxxxx/
>
> If you go look at where this function is called from, you'll see that it's a
> pointer to a stack variable:
>
>
> ---8<---
> static int walk_pte_range_inner(pte_t *pte, unsigned long addr,
> 				unsigned long end, struct mm_walk *walk)
> {
> 	const struct mm_walk_ops *ops = walk->ops;
> 	int err = 0;
>
> 	for (;;) {
> 		if (ops->install_pte && pte_none(ptep_get(pte))) {
> 			pte_t new_pte;
>
> 			err = ops->install_pte(addr, addr + PAGE_SIZE, &new_pte,
> 					       walk);
> ---8<---
>
> I agree that it's extremely confusing. Perhaps, at a minimum, we should come up
> with some kind of naming convention for this and update this and the other
> couple of places that pass pointers to stack-based pXX_t around?
>
> e.g. instead of calling it "ptep", call it "ptevalp" or something like that?

Not sure that'd clarify, we already have a bit of an inconsistent mess with all
this :(

Given it's a stack variable I'm not sure using a helper is in any way helpful
other than I suppose to account for people grepping around for incorrect page
table manipulation code?

>
> Thanks,
> Ryan
>
>
> >  	(*nr_pages)++;
> >
> >  	return 0;
>

Cheers, Lorenzo