mm/vma: rename is_vma_write_only(), separate out shared refcount put

mm/vma: rename is_vma_write_only(), separate out shared refcount put

The is_vma_writer_only() function is misnamed - this isn't determining if
there is only a write lock, as it checks for the presence of the
VM_REFCNT_EXCLUDE_READERS_FLAG.

Really, it is checking to see whether readers are excluded, with a
possibility of a false positive in the case of a detachment (there we
expect the vma->vm_refcnt to eventually be set to
VM_REFCNT_EXCLUDE_READERS_FLAG, whereas for an attached VMA we expect it
to eventually be set to VM_REFCNT_EXCLUDE_READERS_FLAG + 1).

Rename the function accordingly.

Relatedly, we use a __refcount_dec_and_test() primitive directly in
vma_refcount_put(), using the old value to determine what the reference
count ought to be after the operation is complete (ignoring racing
reference count adjustments).

Wrap this into a __vma_refcount_put_return() function, which we can then
utilise in vma_mark_detached() and thus keep the refcount primitive usage
abstracted.

This function, as the name implies, returns the value after the reference
count has been updated.

This reduces duplication in the two invocations of this function.

Also adjust comments, removing duplicative comments covered elsewhere and
adding more to aid understanding.

No functional change intended.

Link: https://lkml.kernel.org/r/32053580bff460eb1092ef780b526cefeb748bad.1769198904.git.lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Waiman Long <longman@redhat.com>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>