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

[Ryan Roberts]

On 07/03/2025 13:04, Lorenzo Stoakes wrote:
> 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@xxxxxxxxxxxxxxx>
>> Fixes: 662df3e5c376 ("mm: madvise: implement lightweight guard page mechanism")
>> Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
>> ---
>>  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

Ahh my bad. In that case, please ignore the patch.

But out of interest, why are you doing it like this? I find it a bit confusing
as all the other ops (e.g. pte_entry()) work directly on the pgtable's pte
without the intermediate.

Thanks,
Ryan

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