mm: pgtable: correct the wrong comment about ptdesc->__page_flags

Patch series "minor fixes and supplement for ptdesc".

In this series, the [PATCH 1/3] and [PATCH 2/3] are fixes for some issues
discovered during code inspection.

The [PATCH 3/3] is a supplement to ptdesc conversion in s390, I don't know
why this is not done in the commit 6326c26c ("s390: convert various
pgalloc functions to use ptdescs"), maybe I missed something.  And since I
don't have an s390 environment, I hope kernel test robot can help compile
and test, and this is why I did not fold [PATCH 2/3] and [PATCH 3/3] into
one patch.


This patch (of 3):

The commit 32cc0b7c ("powerpc: add pte_free_defer() for pgtables
sharing page") introduced the use of PageActive flag to page table
fragments tracking, so the ptdesc->__page_flags is not unused, so correct
the wrong comment.

Link: https://lkml.kernel.org/r/cover.1709541697.git.zhengqi.arch@bytedance.com
Link: https://lkml.kernel.org/r/cc42d5915fd98fd802f920de243f535efcfe01db.1709541697.git.zhengqi.arch@bytedance.comSigned-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Mike Rapoport (IBM) <rppt@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Vishal Moola (Oracle) <vishal.moola@gmail.com>
Cc: Christian Borntraeger <borntraeger@linux.ibm.com>
Cc: Claudio Imbrenda <imbrenda@linux.ibm.com>
Cc: Janosch Frank <frankja@linux.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/mm_types.h

@@ -419,7 +419,7 @@ FOLIO_MATCH(compound_head, _head_2a);
 
 /**
  * struct ptdesc -    Memory descriptor for page tables.
- * @__page_flags:     Same as page flags. Unused for page tables.
+ * @__page_flags:     Same as page flags. Powerpc only.
  * @pt_rcu_head:      For freeing page table pages.
  * @pt_list:          List of used page tables. Used for s390 and x86.
  * @_pt_pad_1:        Padding that aliases with page's compound head.