mm: rework compound_head() for power-of-2 sizeof(struct page)

mm: rework compound_head() for power-of-2 sizeof(struct page)

For tail pages, the kernel uses the 'compound_info' field to get to the
head page.  The bit 0 of the field indicates whether the page is a tail
page, and if set, the remaining bits represent a pointer to the head page.

For cases when size of struct page is power-of-2, change the encoding of
compound_info to store a mask that can be applied to the virtual address
of the tail page in order to access the head page.  It is possible because
struct page of the head page is naturally aligned with regards to order of
the page.

The significant impact of this modification is that all tail pages of the
same order will now have identical 'compound_info', regardless of the
compound page they are associated with.  This paves the way for
eliminating fake heads.

The HugeTLB Vmemmap Optimization (HVO) creates fake heads and it is only
applied when the sizeof(struct page) is power-of-2.  Having identical tail
pages allows the same page to be mapped into the vmemmap of all pages,
maintaining memory savings without fake heads.

If sizeof(struct page) is not power-of-2, there is no functional changes.

Limit mask usage to HugeTLB vmemmap optimization (HVO) where it makes a
difference.  The approach with mask would work in the wider set of
conditions, but it requires validating that struct pages are naturally
aligned for all orders up to the MAX_FOLIO_ORDER, which can be tricky.

Link: https://lkml.kernel.org/r/20260227194302.274384-8-kas@kernel.org
Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
Reviewed-by: Muchun Song <muchun.song@linux.dev>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Acked-by: Usama Arif <usamaarif642@gmail.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: Alexandre Ghiti <alex@ghiti.fr>
Cc: Baoquan He <bhe@redhat.com>
Cc: Christoph Lameter <cl@gentwo.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Frank van der Linden <fvdl@google.com>
Cc: Harry Yoo <harry.yoo@oracle.com>
Cc: Huacai Chen <chenhuacai@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: WANG Xuerui <kernel@xen0n.name>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>