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

[Zi Yan <ziy@nvidia.com>]

On 13 Nov 2025, at 16:56, Balbir Singh wrote:

> On 11/14/25 08:45, Zi Yan wrote:
>> On 13 Nov 2025, at 16:39, Balbir Singh wrote:
>>
>>> On 11/13/25 10:49, Balbir Singh wrote:
>>>> On 11/12/25 22:34, David Hildenbrand (Red Hat) wrote:
>>>>> 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.
>>>>>
>>>>
>>>> Ack
>>>>
>>>>>>
>>>>>>
>>>>>>>>
>>>>>>>> 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.
>>>>>
>>>>
>>>> I came up with a helper, but that ends up with another boolean do_lru.
>>>>
>>>>
>>>
>>> Zi, David, any opinions on the approach below?
>>
>> Looks good to me. We might want a better name instead of
>> __folio_split_unmapped(). Or __split_unmapped_folio() should
>> be renamed, since these two function names are too similar.
>>
>> Maybe __folio_split_unmapped() -> __freeze_and_split_unmapped_folio().
>>
>> Feel free to come up with a better name. :)
>>
>
> I had __folio_split_unfreeze() to indicate we split the folio and unfreeze at the end, but
> it does not reflect that we freeze it as well. Looks like we are trending towards folio_split
> as the prefix (IIUC Dave correctly), I like your name, but if folio_split is going to be
> required then may be __folio_split_unmapped_unfreeze()?
>

OK, if __folio prefix is a convention, how about
__folio_freeze_and_split_unmapped()? __folio_split_unmapped_unfreeze() sounds
like folio is frozen when the function is called. Of course, a more accurate
one would be __folio_freeze_split_unfreeze_unmapped(). It can work if
it is not too long. :)


Best Regards,
Yan, Zi