Re: [PATCH] mm: Always initialise folio->_deferred_list

[Matthew Wilcox <willy@xxxxxxxxxxxxx>]

On Tue, Mar 12, 2024 at 02:17:01PM +0100, David Hildenbrand wrote:
> On 12.03.24 04:50, Matthew Wilcox (Oracle) wrote:
> > +++ b/mm/page_alloc.c
> > @@ -1006,10 +1006,11 @@ static int free_tail_page_prepare(struct page *head_page, struct page *page)
> >   		}
> >   		break;
> >   	case 2:
> > -		/*
> > -		 * the second tail page: ->mapping is
> > -		 * deferred_list.next -- ignore value.
> > -		 */
> > +		/* the second tail page: deferred_list overlaps ->mapping */
> > +		if (unlikely(!list_empty(&folio->_deferred_list))) {
> > +			bad_page(page, "on deferred list");
> > +			goto out;
> > +		}
> 
> IIRC, hugetlb might overwrite this with _hugetlb_subpool? Isn't that a
> problem we have to handle?

Ah yes, you're right.  I think this should do the trick?

+++ b/mm/hugetlb.c
@@ -1796,6 +1796,7 @@ static void __update_and_free_hugetlb_folio(struct hstate *h,
                destroy_compound_gigantic_folio(folio, huge_page_order(h));
                free_gigantic_folio(folio, huge_page_order(h));
        } else {
+               INIT_LIST_HEAD(&folio->_deferred_list);
                __free_pages(&folio->page, huge_page_order(h));
        }
 }

(that should also be a folio_put(), IMO)