Re: [PATCH v1 02/11] mm/pagewalk: introduce folio_walk_start() + folio_walk_end()

David Hildenbrand
2024-08-07T09:32:02Z

On 07.08.24 11:17, Claudio Imbrenda wrote:
> On Fri, 2 Aug 2024 17:55:15 +0200
> David Hildenbrand <david@redhat.com> wrote:
>
>> We want to get rid of follow_page(), and have a more reasonable way to
>> just lookup a folio mapped at a certain address, perform some checks while
>> still under PTL, and then only conditionally grab a folio reference if
>> really required.
>>
>> Further, we might want to get rid of some walk_page_range*() users that
>> really only want to temporarily lookup a single folio at a single address.
>>
>> So let's add a new page table walker that does exactly that, similarly
>> to GUP also being able to walk hugetlb VMAs.
>>
>> Add folio_walk_end() as a macro for now: the compiler is not easy to
>> please with the pte_unmap()->kunmap_local().
>>
>> Note that one difference between follow_page() and get_user_pages(1) is
>> that follow_page() will not trigger faults to get something mapped. So
>> folio_walk is at least currently not a replacement for get_user_pages(1),
>> but could likely be extended/reused to achieve something similar in the
>> future.
>

[...]

>> +pmd_table:
>> + VM_WARN_ON_ONCE(pud_leaf(*pudp));
>

Thanks for the review!

> is this warning necessary? can this actually happen?
> and if it can happen, wouldn't it be more reasonable to return NULL?

The we have to turn this into an unconditional WARN_ON_ONCE() that
cannot be compiled out.

It's something that should be found early during testing (like I had a
bug where I misspelled "CONFIG_PGTABLE_HAS_HUGE_LEAVES" above that took
me 2h to debug, so I added it ;) ), and shouldn't need runtime checks.

Same for the other one.

Thanks!

--
Cheers,

David / dhildenb