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

[Balbir Singh <balbirs@nvidia.com>]

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()?



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Balbir