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

[Matthew Wilcox <willy@infradead.org>]

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