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

[David Hildenbrand (Red Hat)]

On 11/23/25 19:38, Barry Song wrote:
> Hi Zi Yan,
>
> Thanks for the nice cleanup.
>
> On Sat, Nov 22, 2025 at 10:55 AM Zi Yan <ziy@xxxxxxxxxx> 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@xxxxxxxxxx>
> > ---
> >   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.

Could we get rid of the "warns" parameter and simply always do a 
pr_warn_once()?

As an alternative, simply move the warning to the single caller

VM_WARN_ONCE(ret == -EINVAL, "Tried to split an unsplittable folio");

--
Cheers

David