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

["David Hildenbrand (Red Hat)" <david@kernel.org>]

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@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.

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