mm/page_alloc: remove set_page_private() in prep_compound_tail()

Tail pages are expected to have (and optionally be checked) zeroed
->private when they are freed.  It stays true during subsequent
reallocation, so replace the tail_page->private initialization with a
VM_WARN_ON_ONCE() in compound page preparation.

Link: https://lore.kernel.org/20260709-keep-subpage-private-zero-at-free-v3-5-7e4fe155f5b9@nvidia.com
Signed-off-by: Zi Yan <ziy@nvidia.com>
Acked-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Lance Yang <lance.yang@linux.dev>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Brendan Jackman <jackmanb@google.com>
Cc: Brendan Jackman <brendan.jackman@linux.dev>
Cc: Dennis Zhou <dennis@kernel.org>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Nico Pache <npache@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/internal.h

@@ -741,7 +741,7 @@ static inline void prep_compound_tail(struct page *tail,
 {
 	tail->mapping = TAIL_MAPPING;
 	set_compound_head(tail, head, order);
-	set_page_private(tail, 0);
+	VM_WARN_ON_ONCE(tail->private);
 }
 
 static inline void init_compound_tail(struct page *tail,