Re: [PATCH 14/22] mm: set page table page type when allocated

[Kevin Brodsky]

On 20/07/2026 21:05, Vishal Moola wrote:
> On Thu, Jul 16, 2026 at 11:49:31AM +0200, Kevin Brodsky wrote:
>>> We should switch over from __folio_set_pgtable() to __SetPagePgtable()
>>> in this patch. Ptdescs and folios should be treated as distinctly
>>> independent of each other.
>> I sure remember the previous discussion, but my reading of your comments
>> [1] is that we cannot really change this as long as pagetable_alloc()
>> can be called with order > 0, right? Multiple archs still rely on that.
> Kinda...
>
> The concern was about callers accessing tail pages directly for that
> information. Using the folio API ensures that we always look into the
> head page.
>
> Since we're now doing it at allocation time, we know we always have the
> head page, so it's fine.

Ah got it, that makes sense.

>
> The dtor/free should be more questionable because the underlying api
> uses virt_to_page() which could look at a tail page... oopsie. I
> haven't seen any reports tied to it so its probably fine (the
> setting/clearing has been using the folio API since ptdescs).

Right, that should be fine - calling pagetable_free() on anything else
than a ptdesc allocated by pagetable_alloc() is a bug, so if we get any
crash because of this we should just go and fix the underlying issue.
I'd still switch to __SetPagePgtable() in a separate commit to improve
bisectability.

FWIW there are definitely cases where pagetable_free() and
pagetable_alloc() calls aren't properly matched (see my reply to the
cover letter), this needs to be addressed for this series anyway.

> We can say thats a happy accident that makes distinguishing ptdescs
> and folios easier :)

Indeed!

- Kevin