Re: [PATCH v2 1/2] mm: Allow lockless kernel pagetable walking

[David Hildenbrand <david@redhat.com>]

On 10.06.25 15:35, Lorenzo Stoakes wrote:
> On Tue, Jun 10, 2025 at 03:31:56PM +0200, David Hildenbrand wrote:
>> On 10.06.25 15:27, Lorenzo Stoakes wrote:
>>> On Tue, Jun 10, 2025 at 03:24:16PM +0200, David Hildenbrand wrote:
>>>> On 10.06.25 14:07, Lorenzo Stoakes wrote:
>>>>> OK so I think the best solution here is to just update check_ops_valid(), which
>>>>> was kind of sucky anyway (we check everywhere but walk_page_range_mm() to
>>>>> enforce the install pte thing).
>>>>>
>>>>> Let's do something like:
>>>>>
>>>>> #define OPS_MAY_INSTALL_PTE	(1<<0)
>>>>> #define OPS_MAY_AVOID_LOCK	(1<<1)
>>>>>
>>>>> and update check_ops_valid() to take a flags or maybe 'capabilities' field.
>>>>>
>>>>> Then check based on this e.g.:
>>>>>
>>>>> if (ops->install_pte && !(capabilities & OPS_MAY_INSTALL_PTE))
>>>>> 	return false;
>>>>>
>>>>> if (ops->walk_lock == PGWALK_NOLOCK && !(capabilities & OPS_MAY_AVOID_LOCK))
>>>>> 	return false;
>>>>>
>>>>
>>>> Hm. I mean, we really only want to allow this lockless check for
>>>> walk_kernel_page_table_range(), right?
>>>>
>>>> Having a walk_kernel_page_table_range_lockeless() might (or might not) be
>>>> better, to really only special-case this specific path.
>>>
>>> Agree completely, Dev - let's definitely do this.
>>>
>>>>
>>>> So, I am wondering if we should further start splitting the
>>>> kernel-page-table walker up from the mm walker, at least on the "entry"
>>>> function for now.
>>>
>>> How do you mean?
>>
>> In particular, "struct mm_walk_ops"
>>
>> does not quite make sense when not actually walking a "real" mm .
>>
>> So maybe we should start having a separate structure where *vma,
>> install_pte, walk_lock, hugetlb* does not even exist.
>>
>> It might be a bit of churn, though ... not sure if there could be an easy
>> translation layer for now.
> 
> But you know... I looove churn right? <3 <3 <3 :)))
> 
> That's a nice idea, but I think something that should be a follow up.

Yes, absolutely, just wanted to raise it :)

-- 
Cheers,

David / dhildenb