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

[Vishal Moola <vishal.moola@gmail.com>]

On Thu, Jul 16, 2026 at 11:49:31AM +0200, Kevin Brodsky wrote:
> On 14/07/2026 17:16, Vishal Moola wrote:
> > On Tue, Jul 14, 2026 at 03:04:03PM +0100, Kevin Brodsky wrote:
> >> As a step towards removing pagetable ctor/dtor calls for kernel
> >> mm's, move the page type updates from the ctor/dtor to
> >> pagetable_{alloc,free}.
> > Thanks for doing this patchset.
> 
> Thanks for suggesting it :) It's turning out to be quite a bit more
> involved than I expected though! (There are multiple cases where we call
> the ctor/dtor without using pagetable_{alloc,free}, see my reply to the
> cover letter.)
> 
> > 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.

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).

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

> > Eventually, we want to replace the memcg accounting as well, but
> > there's no existing API for that right now. Lets leave that as is for
> > now; this patchset is big enough already ;)
> 
> Yep, looks like there was an attempt to improve this but it had to be
> reverted [2].
> 
> - Kevin
> 
> [1] https://lore.kernel.org/all/aha321ndGSdReBJR@fedora/
> [2]
> https://lore.kernel.org/all/20260225002434.2953895-1-axelrasmussen@google.com/
>