Re: [PATCH v3 16/18] mm/huge_memory: clarify supported split orders in comment

[Kairui Song]

On Wed, Aug 26, 2026 at 10:02 AM Zi Yan <ziy@xxxxxxxxxx> wrote:
>
> On Thu Aug 20, 2026 at 2:55 PM EDT, Kairui Song via B4 Relay wrote:
> > From: Kairui Song <kasong@xxxxxxxxxxx>
> >
> > The doc comment for __split_huge_page_to_list_to_order() needs an
> > update: only order 1 is rejected for anon and swapcache folios,
> > matching the new_order == 1 check in folio_check_splittable().
> >
> > Also realign the continuation line of the function signature while at
> > it.
> >
> > Signed-off-by: Kairui Song <kasong@xxxxxxxxxxx>
> > ---
> >  mm/huge_memory.c | 11 +++++------
> >  1 file changed, 5 insertions(+), 6 deletions(-)
> >
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index 06f353f937d1..0a971ca48151 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -4400,11 +4400,10 @@ int folio_split_unmapped(struct folio *folio, unsigned int new_order)
> >   *    GUP pins, will result in the folio not getting split; instead, the caller
> >   *    will receive an -EAGAIN.
> >   *
> > - * 4) @new_order > 1, usually. Order-1 is not supported for anon or swapcache
> > - *    folios: anon folios need subpage 2 for _deferred_list, which order-1
> > - *    folios lack, and a swapcache folio may become anon once faulted in.
> > - *    File-backed order-1 folios are supported, since they do not use
> > - *    _deferred_list.
> > + * 4) @new_order != 1 for anon or swapcache. Anon folios need subpage 2 for
>
> We use head page + tail pages instead of subpages, so the sentence could
> be
>
> Anon folios stores _deferred_list in second tail page, which order-1
> folios lack, ...

Good to know, will update.
>
> > + *    _deferred_list, which order-1 folios lack, and a swapcache folio may
> > + *    become anon once faulted in. File-backed order-1 folios are supported,
> > + *    since they do not use _deferred_list.
> >   *
> >   * After splitting, the caller's folio reference will be transferred to @page,
> >   * resulting in a raised refcount of @page after this call. The other pages may
> > @@ -4432,7 +4431,7 @@ int folio_split_unmapped(struct folio *folio, unsigned int new_order)
> >   * with the folio. Splitting to order 0 is compatible with all folios.
> >   */
> >  int __split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
> > -                                  unsigned int new_order)
> > +                                    unsigned int new_order)
> >  {
> >       struct folio *folio = page_folio(page);
> >
>
> Otherwise, LGTM.
>
> Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>
>

Thanks!