Re: [PATCH] mm/huge_memory.c: introduce split_unmapped_folio_to_order

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

On 12.11.25 11:17, Balbir Singh wrote:
> On 11/12/25 21:00, David Hildenbrand (Red Hat) wrote:
>> On 12.11.25 05:46, Balbir Singh wrote:
>>> Unmapped was added as a parameter to __folio_split() and related
>>> call sites to support splitting of folios already in the midst
>>> of a migration. This special case arose for device private folio
>>> migration since during migration there could be a disconnect between
>>> source and destination on the folio size.
>>>
>>> Introduce split_unmapped_folio_to_order() to handle this special case.
>>> This in turn removes the special casing introduced by the unmapped
>>> parameter in __folio_split().
>>
>> As raised recently, I would hope that we can find a way to make all these splitting functions look more similar in the long term, ideally starting with "folio_split" / "folio_try_split".
>>
>> What about
>>
>>      folio_split_unmapped()
>>
>> Do we really have to spell out the "to order" part in the function name?
>>
>> And if it's more a mostly-internal helper, maybe
>>
>>      __folio_split_unmapped()
>>
>> subject: "mm/huge_memory: introduce ..."
>>
> 
> I can rename it, but currently it confirms to the split_folio with order in the name
> The order is there in the name because in the future with mTHP we will want to
> support splitting to various orders.

I think we should start naming them more consistently regarding 
folio_split() immediately and cleanup the other ones later.

I don't understand why "_to_order" must be in the name right now. You 
can add another variant and start using longer names when really required.

> 
> 
>>>
>>> Cc: Andrew Morton <akpm@linux-foundation.org>
>>> Cc: David Hildenbrand <david@redhat.com>
>>> Cc: Zi Yan <ziy@nvidia.com>
>>> Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
>>> Cc: Rakie Kim <rakie.kim@sk.com>
>>> Cc: Byungchul Park <byungchul@sk.com>
>>> Cc: Gregory Price <gourry@gourry.net>
>>> Cc: Ying Huang <ying.huang@linux.alibaba.com>
>>> Cc: Alistair Popple <apopple@nvidia.com>
>>> Cc: Oscar Salvador <osalvador@suse.de>
>>> Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
>>> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
>>> Cc: "Liam R. Howlett" <Liam.Howlett@oracle.com>
>>> Cc: Nico Pache <npache@redhat.com>
>>> Cc: Ryan Roberts <ryan.roberts@arm.com>
>>> Cc: Dev Jain <dev.jain@arm.com>
>>> Cc: Barry Song <baohua@kernel.org>
>>> Cc: Lyude Paul <lyude@redhat.com>
>>> Cc: Danilo Krummrich <dakr@kernel.org>
>>> Cc: David Airlie <airlied@gmail.com>
>>> Cc: Simona Vetter <simona@ffwll.ch>
>>> Cc: Ralph Campbell <rcampbell@nvidia.com>
>>> Cc: Mika Penttilä <mpenttil@redhat.com>
>>> Cc: Matthew Brost <matthew.brost@intel.com>
>>> Cc: Francois Dugast <francois.dugast@intel.com>
>>>
>>> Suggested-by: Zi Yan <ziy@nvidia.com>
>>> Signed-off-by: Balbir Singh <balbirs@nvidia.com>
>>> ---
>>>    include/linux/huge_mm.h |   5 +-
>>>    mm/huge_memory.c        | 135 ++++++++++++++++++++++++++++++++++------
>>>    mm/migrate_device.c     |   3 +-
>>>    3 files changed, 120 insertions(+), 23 deletions(-)
>>>
>>> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
>>> index e2e91aa1a042..9155e683c08a 100644
>>> --- a/include/linux/huge_mm.h
>>> +++ b/include/linux/huge_mm.h
>>> @@ -371,7 +371,8 @@ enum split_type {
>>>      bool can_split_folio(struct folio *folio, int caller_pins, int *pextra_pins);
>>>    int __split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
>>> -        unsigned int new_order, bool unmapped);
>>> +        unsigned int new_order);
>>> +int split_unmapped_folio_to_order(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,
>>> @@ -382,7 +383,7 @@ int folio_split(struct folio *folio, unsigned int new_order, struct page *page,
>>>    static inline int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
>>>            unsigned int new_order)
>>>    {
>>> -    return __split_huge_page_to_list_to_order(page, list, new_order, false);
>>> +    return __split_huge_page_to_list_to_order(page, list, new_order);
>>>    }
>>>    static inline int split_huge_page_to_order(struct page *page, unsigned int new_order)
>>>    {
>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>> index 0184cd915f44..942bd8410c54 100644
>>> --- a/mm/huge_memory.c
>>> +++ b/mm/huge_memory.c
>>> @@ -3747,7 +3747,6 @@ bool folio_split_supported(struct folio *folio, unsigned int new_order,
>>>     * @lock_at: a page within @folio to be left locked to caller
>>>     * @list: after-split folios will be put on it if non NULL
>>>     * @split_type: perform uniform split or not (non-uniform split)
>>> - * @unmapped: The pages are already unmapped, they are migration entries.
>>>     *
>>>     * It calls __split_unmapped_folio() to perform uniform and non-uniform split.
>>>     * It is in charge of checking whether the split is supported or not and
>>> @@ -3763,7 +3762,7 @@ bool folio_split_supported(struct folio *folio, unsigned int new_order,
>>>     */
>>>    static int __folio_split(struct folio *folio, unsigned int new_order,
>>>            struct page *split_at, struct page *lock_at,
>>> -        struct list_head *list, enum split_type split_type, bool unmapped)
>>> +        struct list_head *list, enum split_type split_type)
>>
>> Yeah, nice to see that go.
>>
>>>    {
>>>        struct deferred_split *ds_queue;
>>>        XA_STATE(xas, &folio->mapping->i_pages, folio->index);
>>> @@ -3809,14 +3808,12 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>>>             * is taken to serialise against parallel split or collapse
>>>             * operations.
>>>             */
>>> -        if (!unmapped) {
>>> -            anon_vma = folio_get_anon_vma(folio);
>>> -            if (!anon_vma) {
>>> -                ret = -EBUSY;
>>> -                goto out;
>>> -            }
>>> -            anon_vma_lock_write(anon_vma);
>>> +        anon_vma = folio_get_anon_vma(folio);
>>> +        if (!anon_vma) {
>>> +            ret = -EBUSY;
>>> +            goto out;
>>>            }
>>> +        anon_vma_lock_write(anon_vma);
>>>            mapping = NULL;
>>>        } else {
>>>            unsigned int min_order;
>>> @@ -3882,8 +3879,7 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>>>            goto out_unlock;
>>>        }
>>>    -    if (!unmapped)
>>> -        unmap_folio(folio);
>>> +    unmap_folio(folio);
>>>    
>>
>> Hm, I would have hoped that we could factor out the core logic and reuse it for the new helper, instead of duplicating code.
>>
>> Did you look into that?
>>
>>
> 
> I did, I ended up with larger spaghetti, I was hoping to look it as a follow up
> after the series with the mTHP changes and support (that is to be designed and
> prototyped).

Looking at it in more detail, the code duplication is not desired.

We have to find a way to factor the existing code out and reuse it from 
any new function.

-- 
Cheers

David