mm: Copy-on-Write (COW) reuse support for PTE-mapped THP

mm: Copy-on-Write (COW) reuse support for PTE-mapped THP

Currently, we never end up reusing PTE-mapped THPs after fork.  This
wasn't really a problem with PMD-sized THPs, because they would have to be
PTE-mapped first, but it's getting a problem with smaller THP sizes that
are effectively always PTE-mapped.

With our new "mapped exclusively" vs "maybe mapped shared" logic for large
folios, implementing CoW reuse for PTE-mapped THPs is straight forward: if
exclusively mapped, make sure that all references are from these (our)
mappings.  Add some helpful comments to explain the details.

CONFIG_TRANSPARENT_HUGEPAGE selects CONFIG_MM_ID.  If we spot an anon
large folio without CONFIG_TRANSPARENT_HUGEPAGE in that code, something is
seriously messed up.

There are plenty of things we can optimize in the future: For example, we
could remember that the folio is fully exclusive so we could speedup the
next fault further.  Also, we could try "faulting around", turning
surrounding PTEs that map the same folio writable.  But especially the
latter might increase COW latency, so it would need further investigation.

Link: https://lkml.kernel.org/r/20250303163014.1128035-14-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Cc: Andy Lutomirks^H^Hski <luto@kernel.org>
Cc: Borislav Betkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jann Horn <jannh@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Lance Yang <ioworker0@gmail.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Matthew Wilcow (Oracle) <willy@infradead.org>
Cc: Michal Koutn <mkoutny@suse.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: tejun heo <tj@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Zefan Li <lizefan.x@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>