On Thu, Jul 16, 2026 at 11:49:31AM +0200, Kevin Brodsky wrote:
On 14/07/2026 17:16, Vishal Moola wrote:
quoted
On Tue, Jul 14, 2026 at 03:04:03PM +0100, Kevin Brodsky wrote:
quoted
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.)
quoted
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 :)
quoted
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/ (local)
[2]
https://lore.kernel.org/all/20260225002434.2953895-1-axelrasmussen@google.com/ (local)