mm, rmap: Add yet more comments to page_get_anon_vma/page_lock_anon_vma

 * The page might have been remapped to a different anon_vma or the anon_vma

 * returned may already be freed (and even reused).

 *

 * In case it was remapped to a different anon_vma, the new anon_vma will be a

 * child of the old anon_vma, and the anon_vma lifetime rules will therefore

 * ensure that any anon_vma obtained from the page will still be valid for as

 * long as we observe page_mapped() [ hence all those page_mapped() tests ].

 *

 * All users of this function must be very careful when walking the anon_vma

 * chain and verify that the page in question is indeed mapped in it

 * [ something equivalent to page_mapped_in_vma() ].

		/*

		 * If the page is still mapped, then this anon_vma is still

		 * its anon_vma, and holding the mutex ensures that it will

		 * not go away, see __put_anon_vma().

		 * not go away, see anon_vma_free().

		 */

		if (!page_mapped(page)) {

			mutex_unlock(&root_anon_vma->mutex);