Re: [PATCH] mm: Fix memblock_free_late() when using deferred struct page

[Alexander Potapenko <glider@google.com>]

On Fri, Feb 6, 2026 at 11:33 AM Mike Rapoport <rppt@kernel.org> wrote:
>
> (added KMSAN folks)
>
> On Wed, Feb 04, 2026 at 08:02:13PM +1100, Benjamin Herrenschmidt wrote:
> > On Wed, 2026-02-04 at 09:39 +0200, Mike Rapoport wrote:
> > > > I might be missing something but I don't see what would restrict
> > > > this
> > > > to the early pre-initialized struct pages other than that
> > > > early_page_initialised() test, so we can't rely on anything in
> > > > struct
> > > > page inside memblock_free_pages().
> > >
> > > Right, we can't rely on PG_Reserved being cleared for uninitialized
> > > pages :/
> > >
> > > But I overlooked an easier and actually reliable way: use
> > > free_reserved_area() instead of memblock_free_late().
> >
> > You mean replace all callers of memblock_free_late() and kill it ?
>
> That would be great, but with all the subtle differences you note below
> it's for the future :)
>
> > Or make memblock_free_late() use free_reserved_area() instead of
> > memblock_free_pages() ? :-)
>
> Yes, I think either calling free_reserved_page() in the loop in
> memblock_free_late() or replacing the entire loop with free_reserved_area().
>
> > The former misses:
> >  - totalram_pages_inc() and kmemleak_free_part_phys() in
> > memblock_free_late()
> >
> > They also both miss as far as I can tell:
> >
> >       if (!kmsan_memblock_free_pages(page, order)) {
> >               /* KMSAN will take care of these pages. */
> >               return;
> >       }
> >
> > But I don't know if that matters, I don't know anything about kmsan :-)
>
> AFAIU, here kmsan allocates metadata for each page freed to buddy, but it
> handles reserved memory differently anyway, so it shouldn't be a problem.

I am a bit late here, sorry.

Yes, kmsan_init_shadow() iterates over the reserved ranges (which I
believe happens before memblock_free_late(), right?) and reserves the
metadata pages for them. So freeing these ranges we won't need to call
kmsan_memblock_free_pages().