[PATCH v2 02/12] x86: pgtable: Always use pte_free_kernel()

[Kevin Brodsky]

On 08/04/2025 19:40, Matthew Wilcox wrote:
> On Tue, Apr 08, 2025 at 09:54:42AM -0700, Dave Hansen wrote:
>> On 4/8/25 09:37, Matthew Wilcox wrote:
>>> On Tue, Apr 08, 2025 at 08:22:47AM -0700, Dave Hansen wrote:
>>>> Are there any tests for folio_test_pgtable() at free_page() time? If we
>>>> had that, it would make it less likely that another free_page() user
>>>> could sneak in without calling the destructor.
>>> It's hidden, but yes:
>>>
>>> static inline bool page_expected_state(struct page *page,
>>>                                         unsigned long check_flags)
>>> {
>>>         if (unlikely(atomic_read(&page->_mapcount) != -1))
>>>                 return false;
>>>
>>> PageTable uses page_type which aliases with mapcount, so this check
>>> covers "PageTable is still set when the last refcount to it is put".
>> Huh, so shouldn't we have ended up in bad_page() for these, other than:
>>
>>         pagetable_dtor(virt_to_ptdesc(pmd));
>>         free_page((unsigned long)pmd);
> I think at this point in Kevin's series, we don't call the ctor for
> these pages, so we never set PageTable() on them. I could be wrong;

Correct, that's why I added this patch early in the series (the next
patch adds the ctor call in pte_alloc_one_kernel()).

The BUG() in v1 was indeed triggered by a page_expected_state() check [1].

> as Kevin says, this is all very twisty and confusing with exceptions and
> exceptions to exceptions.  This series should reduce the confusion.

I hope so!

- Kevin

[1] https://lore.kernel.org/oe-lkp/202503211612.e11bd73f-lkp@intel.com/