mm/mprotect: special-case small folios when applying permissions

mm/mprotect: special-case small folios when applying permissions

The common order-0 case is important enough to want its own branch, and
avoids the hairy, large loop logic that the CPU does not seem to handle
particularly well.

While at it, encourage the compiler to inline batch PTE logic and resolve
constant branches by adding __always_inline strategically.

Link: https://lore.kernel.org/20260402141628.3367596-3-pfalcato@suse.de
Signed-off-by: Pedro Falcato <pfalcato@suse.de>
Suggested-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
Tested-by: Luke Yang <luyang@redhat.com>
Reviewed-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Jann Horn <jannh@google.com>
Cc: Jiri Hladky <jhladky@redhat.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>