Re: [PATCH v3] mm/page_alloc: clear page->private in free_pages_prepare()

[David Hildenbrand (Arm)]

On 2/9/26 12:17, Vlastimil Babka wrote:
> On 2/7/26 23:08, David Hildenbrand (Arm) wrote:
> > > -               /*
> > > -                * page->private should not be set in tail pages. Fix up
> > > and warn once
> > > -                * if private is unexpectedly set.
> > > -                */
> > > -               if (unlikely(new_folio->private)) {
> > > -                       VM_WARN_ON_ONCE_PAGE(true, new_head);
> > > -                       new_folio->private = NULL;
> > > -               }
> >
> > BTW, I wonder whether we should bring that check back for non-device folios.
>
> If the rule is now that when upon freeing in free_pages_prepare() we clear
> private in the head page and not tail pages (where we expect the owner of
> the page to do it), maybe that check for tail pages should be done in the
> is_check_pages_enabled() part of free_pages_prepare().
>
> Or should the check be also in the split path because somebody can set a
> tail private between allocation and split? (and not just inherit it from a
> previous allocation that didn't clear it?).

We ran into that check in the past, when folio->X overlayed 
page->private in a tail, and would actually have to be zeroed out.

So it should be part of this splitting code I think.

--
Cheers,

David