Re: [PATCH 0/4] mm: page_ext: Introduce new iteration API

From: Luiz Capitulino
Date: Thu Feb 20 2025 - 15:24:23 EST


On 2025-02-20 05:49, David Hildenbrand wrote:
On 20.02.25 00:52, Andrew Morton wrote:
On Tue, 18 Feb 2025 21:17:46 -0500 Luiz Capitulino <luizcap@xxxxxxxxxx> wrote:

To fix this, this series introduces a new iteration API for page extension
objects. The API checks if the next page extension object can be retrieved
from the current section or if it needs to look up for it in another
section.

...

A regression since 6.12, so we should backport the fix.

...

  include/linux/page_ext.h | 67 +++++++++++++++++++++++++++++++++++++---
  mm/page_ext.c            | 48 ++++++++++++++++++++++++++++
  mm/page_owner.c          | 61 +++++++++++++++++-------------------
  mm/page_table_check.c    | 39 +++++++----------------
  4 files changed, 152 insertions(+), 63 deletions(-)

That's a lot to backport!

Is there some quick-n-dirty fixup we can apply for the sake of -stable
kernels, then work on this long-term approach for future kernels?

I assume we could loop in reset_page_owner()/page_table_check_free()/set_page_owner()/page_table_check_alloc(). Not-so-nice for upstream, maybe good-enough for stable. Still nasty :)

I think Andrew wants to have the quick-n-dirty fix for upstream, so that
it's easier to backport to -stable. Then we work on this solution on top.

OTOH, we don't really expect a lot of conflicts.

Yes, I was able to apply this series on top of 6.12.15 without conflicts.
Given that -stable does backport a lot of fixes anyways, I would push for
having this on -stable.

But just to answer the original question: I can't think of quick-n-dirty,
but I can think of easy-n-ugly:

1. We could add a check for MAX_PAGE_ORDER for the first function in a
call chain calling page_ext_next() (that is, bail out if > MAX_PAGE_ORDER)

2. We could replace all page_ext_next() calls to a version of look_page_ext()
that takes a PFN

But all these ideas have regression risk as well, so I don't see the advantage.