mm/rmap: remove unnecessary root lock dance in anon_vma clone, unmap

mm/rmap: remove unnecessary root lock dance in anon_vma clone, unmap

The root anon_vma of all anon_vma's linked to a VMA must by definition be
the same - a VMA and all of its descendants/ancestors must exist in the
same CoW chain.

Commit bb4aa39676f7 ("mm: avoid repeated anon_vma lock/unlock sequences in
anon_vma_clone()") introduced paranoid checking of the root anon_vma
remaining the same throughout all AVC's in 2011.

I think 15 years later we can safely assume that this is always the case.

Additionally, since unfaulted VMAs being cloned from or unlinked are
no-op's, we can simply lock the anon_vma's associated with this rather
than doing any specific dance around this.

This removes unnecessary checks and makes it clear that the root anon_vma
is shared between all anon_vma's in a given VMA's anon_vma_chain.

Link: https://lkml.kernel.org/r/838030d2f0772b99fa99ff4b4fd571353f14a1a9.1768746221.git.lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Barry Song <v-songbaohua@oppo.com>
Cc: Chris Li <chriscli@google.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: Harry Yoo <harry.yoo@oracle.com>
Cc: Jann Horn <jannh@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Pedro Falcato <pfalcato@suse.de>
Cc: Rik van Riel <riel@surriel.com>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>