Re: [PATCH v2 1/4] mm/huge_memory: change folio_split_supported() to folio_check_splittable()

[Barry Song <21cnbao@gmail.com>]

Hi Zi Yan,

Thanks for the nice cleanup.

On Sat, Nov 22, 2025 at 10:55 AM Zi Yan <ziy@nvidia.com> wrote:
>
> folio_split_supported() used in try_folio_split_to_order() requires
> folio->mapping to be non NULL, but current try_folio_split_to_order() does
> not check it. There is no issue in the current code, since
> try_folio_split_to_order() is only used in truncate_inode_partial_folio(),
> where folio->mapping is not NULL.
>
> To prevent future misuse, move folio->mapping NULL check (i.e., folio is
> truncated) into folio_split_supported(). Since folio->mapping NULL check
> returns -EBUSY and folio_split_supported() == false means -EINVAL, change
> folio_split_supported() return type from bool to int and return error
> numbers accordingly. Rename folio_split_supported() to
> folio_check_splittable() to match the return type change.
>
> While at it, move is_huge_zero_folio() check and folio_test_writeback()
> check into folio_check_splittable() and add kernel-doc.
>
> Signed-off-by: Zi Yan <ziy@nvidia.com>
> ---
>  include/linux/huge_mm.h | 10 ++++--
>  mm/huge_memory.c        | 74 +++++++++++++++++++++++++----------------
>  2 files changed, 53 insertions(+), 31 deletions(-)
>
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index 1d439de1ca2c..97686fb46e30 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -375,8 +375,8 @@ int __split_huge_page_to_list_to_order(struct page *page, struct list_head *list
>  int folio_split_unmapped(struct folio *folio, unsigned int new_order);
>  int min_order_for_split(struct folio *folio);
>  int split_folio_to_list(struct folio *folio, struct list_head *list);
> -bool folio_split_supported(struct folio *folio, unsigned int new_order,
> -               enum split_type split_type, bool warns);
> +int folio_check_splittable(struct folio *folio, unsigned int new_order,
> +                          enum split_type split_type, bool warns);


It feels a bit odd to have a warns parameter here, especially given that it's
a bool. I understand that in one case we're only checking whether a split is
possible, without actually performing it. In the other case, we are performing
the split, so we must confirm it's valid — otherwise it's a bug.

Could we rename split_type to something more like gfp_flags, where we have
variants such as __GFP_NOWARN or something similar? That would make the code
much more readable.

[...]

>
> @@ -3734,10 +3762,18 @@ bool folio_split_supported(struct folio *folio, unsigned int new_order,
>         if ((split_type == SPLIT_TYPE_NON_UNIFORM || new_order) && folio_test_swapcache(folio)) {
>                 VM_WARN_ONCE(warns,
>                         "Cannot split swapcache folio to non-0 order");
> -               return false;
> +               return -EINVAL;
>         }
>
> -       return true;
> +       if (is_huge_zero_folio(folio)) {
> +               pr_warn_ratelimited("Called split_huge_page for huge zero page\n");
> +               return -EINVAL;
> +       }

However, I don’t quite understand why this doesn’t check warns or why it
isn’t using VM_WARN_ONCE. Why is the zero-huge case different?

Thanks
Barry