mm/rmap: fix two comments related to huge_pmd_unshare()

jira VULN-46930
cve-bf CVE-2024-57883
commit-author David Hildenbrand (Red Hat) <david@kernel.org>
commit a8682d5

PMD page table unsharing no longer touches the refcount of a PMD page
table.  Also, it is not about dropping the refcount of a "PMD page" but
the "PMD page table".

Let's just simplify by saying that the PMD page table was unmapped,
consequently also unmapping the folio that was mapped into this page.

This code should be deduplicated in the future.

Link: https://lkml.kernel.org/r/20251223214037.580860-4-david@kernel.org
Fixes: 59d9094 ("mm: hugetlb: independent PMD page table shared count")
	Signed-off-by: David Hildenbrand (Red Hat) <david@kernel.org>
	Reviewed-by: Rik van Riel <riel@surriel.com>
	Tested-by: Laurence Oberman <loberman@redhat.com>
	Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
	Acked-by: Oscar Salvador <osalvador@suse.de>
	Cc: Liu Shixin <liushixin2@huawei.com>
	Cc: Harry Yoo <harry.yoo@oracle.com>
	Cc: Lance Yang <lance.yang@linux.dev>
	Cc: "Uschakow, Stanislav" <suschako@amazon.de>
	Cc: <stable@vger.kernel.org>
	Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
(cherry picked from commit a8682d5)
	Signed-off-by: Marcin Wcisło <marcin.wcislo@conclusive.pl>

Author: pvts-mat

mm/rmap.c

@@ -1556,14 +1556,8 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
 					flush_tlb_range(vma,
 						range.start, range.end);
 					/*
-					 * The ref count of the PMD page was
-					 * dropped which is part of the way map
-					 * counting is done for shared PMDs.
-					 * Return 'true' here.  When there is
-					 * no other sharing, huge_pmd_unshare
-					 * returns false and we will unmap the
-					 * actual page and drop map count
-					 * to zero.
+					 * The PMD table was unmapped,
+					 * consequently unmapping the folio.
 					 */
 					page_vma_mapped_walk_done(&pvmw);
 					break;
@@ -1926,14 +1920,8 @@ static bool try_to_migrate_one(struct folio *folio, struct vm_area_struct *vma,
 						range.start, range.end);
 
 					/*
-					 * The ref count of the PMD page was
-					 * dropped which is part of the way map
-					 * counting is done for shared PMDs.
-					 * Return 'true' here.  When there is
-					 * no other sharing, huge_pmd_unshare
-					 * returns false and we will unmap the
-					 * actual page and drop map count
-					 * to zero.
+					 * The PMD table was unmapped,
+					 * consequently unmapping the folio.
 					 */
 					page_vma_mapped_walk_done(&pvmw);
 					break;