On 22 Oct 2025, at 3:16, Balbir Singh wrote:
> On 10/22/25 13:59, Zi Yan wrote:
>> On 21 Oct 2025, at 17:34, Balbir Singh wrote:
>>
>>> On 10/20/25 09:59, Zi Yan wrote:
>>>> On 19 Oct 2025, at 18:49, Balbir Singh wrote:
>>>>
>>>>> On 10/19/25 19:19, Wei Yang wrote:
>>>>>> On Wed, Oct 01, 2025 at 04:57:02PM +1000, Balbir Singh wrote:
>>>>>> [...]
>>>>>>> static int __folio_split(struct folio *folio, unsigned int new_order,
>>>>>>> struct page *split_at, struct page *lock_at,
>>>>>>> - struct list_head *list, bool uniform_split)
>>>>>>> + struct list_head *list, bool uniform_split, bool
>>>>>>> unmapped)
>>>>>>> {
>>>>>>> struct deferred_split *ds_queue =
>>>>>>> get_deferred_split_queue(folio);
>>>>>>> XA_STATE(xas, &folio->mapping->i_pages, folio->index);
>>>>>>> @@ -3765,13 +3757,15 @@ static int __folio_split(struct folio *folio,
>>>>>>> unsigned int new_order,
>>>>>>> * is taken to serialise against parallel split or
>>>>>>> collapse
>>>>>>> * operations.
>>>>>>> */
>>>>>>> - anon_vma = folio_get_anon_vma(folio);
>>>>>>> - if (!anon_vma) {
>>>>>>> - ret = -EBUSY;
>>>>>>> - goto out;
>>>>>>> + if (!unmapped) {
>>>>>>> + anon_vma = folio_get_anon_vma(folio);
>>>>>>> + if (!anon_vma) {
>>>>>>> + ret = -EBUSY;
>>>>>>> + goto out;
>>>>>>> + }
>>>>>>> + anon_vma_lock_write(anon_vma);
>>>>>>> }
>>>>>>> mapping = NULL;
>>>>>>> - anon_vma_lock_write(anon_vma);
>>>>>>> } else {
>>>>>>> unsigned int min_order;
>>>>>>> gfp_t gfp;
>>>>>>> @@ -3838,7 +3832,8 @@ static int __folio_split(struct folio *folio,
>>>>>>> unsigned int new_order,
>>>>>>> goto out_unlock;
>>>>>>> }
>>>>>>>
>>>>>>> - unmap_folio(folio);
>>>>>>> + if (!unmapped)
>>>>>>> + unmap_folio(folio);
>>>>>>>
>>>>>>> /* block interrupt reentry in xa_lock and spinlock */
>>>>>>> local_irq_disable();
>>>>>>> @@ -3925,10 +3920,13 @@ static int __folio_split(struct folio *folio,
>>>>>>> unsigned int new_order,
>>>>>>>
>>>>>>> next = folio_next(new_folio);
>>>>>>>
>>>>>>> + zone_device_private_split_cb(folio, new_folio);
>>>>>>> +
>>>>>>> expected_refs =
>>>>>>> folio_expected_ref_count(new_folio) + 1;
>>>>>>> folio_ref_unfreeze(new_folio, expected_refs);
>>>>>>>
>>>>>>> - lru_add_split_folio(folio, new_folio, lruvec,
>>>>>>> list);
>>>>>>> + if (!unmapped)
>>>>>>> + lru_add_split_folio(folio, new_folio,
>>>>>>> lruvec, list);
>>>>>>>
>>>>>>> /*
>>>>>>> * Anonymous folio with swap cache.
>>>>>>> @@ -3959,6 +3957,8 @@ static int __folio_split(struct folio *folio,
>>>>>>> unsigned int new_order,
>>>>>>> __filemap_remove_folio(new_folio, NULL);
>>>>>>> folio_put_refs(new_folio, nr_pages);
>>>>>>> }
>>>>>>> +
>>>>>>> + zone_device_private_split_cb(folio, NULL);
>>>>>>> /*
>>>>>>> * Unfreeze @folio only after all page cache entries,
>>>>>>> which
>>>>>>> * used to point to it, have been updated with new
>>>>>>> folios.
>>>>>>> @@ -3982,6 +3982,9 @@ static int __folio_split(struct folio *folio,
>>>>>>> unsigned int new_order,
>>>>>>>
>>>>>>> local_irq_enable();
>>>>>>>
>>>>>>> + if (unmapped)
>>>>>>> + return ret;
>>>>>>
>>>>>> As the comment of __folio_split() and
>>>>>> __split_huge_page_to_list_to_order()
>>>>>> mentioned:
>>>>>>
>>>>>> * The large folio must be locked
>>>>>> * After splitting, the after-split folio containing @lock_at remains
>>>>>> locked
>>>>>>
>>>>>> But here we seems to change the prerequisites.
>>>>>>
>>>>>> Hmm.. I am not sure this is correct.
>>>>>>
>>>>>
>>>>> The code is correct, but you are right in that the documentation needs to
>>>>> be updated.
>>>>> When "unmapped", we do want to leave the folios locked after the split.
>>>>
>>>> Sigh, this "unmapped" code needs so many special branches and a different
>>>> locking
>>>> requirement. It should be a separate function to avoid confusions.
>>>>
>>>
>>> Yep, I have a patch for it, I am also waiting on Matthew's feedback, FYI,
>>> here is
>>> a WIP patch that can be applied on top of the series
>>
>> Nice cleanup! Thanks.
>>
>>>
>>> ---
>>> include/linux/huge_mm.h | 5 +-
>>> mm/huge_memory.c | 137 ++++++++++++++++++++++++++++++++++------
>>> mm/migrate_device.c | 3 +-
>>> 3 files changed, 120 insertions(+), 25 deletions(-)
>>>
>>> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
>>> index c4a811958cda..86e1cefaf391 100644
>>> --- a/include/linux/huge_mm.h
>>> +++ b/include/linux/huge_mm.h
>>> @@ -366,7 +366,8 @@ unsigned long thp_get_unmapped_area_vmflags(struct file
>>> *filp, unsigned long add
>>>
>>> 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 uniform_split_supported(struct folio *folio, unsigned int new_order,
>>> @@ -379,7 +380,7 @@ 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);
>>> }
>>>
>>> /*
>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>> index 8c82a0ac6e69..e20cbf68d037 100644
>>> --- a/mm/huge_memory.c
>>> +++ b/mm/huge_memory.c
>>> @@ -3711,7 +3711,6 @@ bool uniform_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
>>> * @uniform_split: 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
>>> @@ -3727,7 +3726,7 @@ bool uniform_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, bool uniform_split, bool unmapped)
>>> + struct list_head *list, bool uniform_split)
>>> {
>>> struct deferred_split *ds_queue;
>>> XA_STATE(xas, &folio->mapping->i_pages, folio->index);
>>> @@ -3777,14 +3776,12 @@ 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;
>>> @@ -3852,8 +3849,7 @@ static int __folio_split(struct folio *folio,
>>> unsigned int new_order,
>>> goto out_unlock;
>>> }
>>>
>>> - if (!unmapped)
>>> - unmap_folio(folio);
>>> + unmap_folio(folio);
>>>
>>> /* block interrupt reentry in xa_lock and spinlock */
>>> local_irq_disable();
>>> @@ -3954,8 +3950,7 @@ static int __folio_split(struct folio *folio,
>>> unsigned int new_order,
>>> expected_refs = folio_expected_ref_count(new_folio) + 1;
>>> folio_ref_unfreeze(new_folio, expected_refs);
>>>
>>> - if (!unmapped)
>>> - lru_add_split_folio(folio, new_folio, lruvec,
>>> list);
>>> + lru_add_split_folio(folio, new_folio, lruvec, list);
>>>
>>> /*
>>> * Anonymous folio with swap cache.
>>> @@ -4011,9 +4006,6 @@ static int __folio_split(struct folio *folio,
>>> unsigned int new_order,
>>>
>>> local_irq_enable();
>>>
>>> - if (unmapped)
>>> - return ret;
>>> -
>>> if (nr_shmem_dropped)
>>> shmem_uncharge(mapping->host, nr_shmem_dropped);
>>>
>>> @@ -4057,6 +4049,111 @@ static int __folio_split(struct folio *folio,
>>> unsigned int new_order,
>>> return ret;
>>> }
>>>
>>> +/*
>>> + * This function is a helper for splitting folios that have already been
>>> unmapped.
>>> + * The use case is that the device or the CPU can refuse to migrate THP
>>> pages in
>>> + * the middle of migration, due to allocation issues on either side
>>> + *
>>> + * The high level code is copied from __folio_split, since the pages are
>>> anonymous
>>> + * and are already isolated from the LRU, the code has been simplified to
>>> not
>>> + * burden __folio_split with unmapped sprinkled into the code.
>>
>> I wonder if it makes sense to remove CPU side folio from both deferred_split
>> queue
>> and swap cache before migration to further simplify
>> split_unmapped_folio_to_order().
>> Basically require that device private folios cannot be on deferred_split
>> queue nor
>> swap cache.
>>
>
> This API can be called for non-device private folios as well. Device private
> folios are
> already not on the deferred queue. The use case is
>
> 1. Migrate a large folio page from CPU to Device
> 2. SRC - CPU has a THP (large folio page)
> 3. DST - Device cannot allocate a large page, hence split the SRC page
Right. That is what I am talking about, sorry I was not clear.
I mean when migrating a large folio from CPU to device, the CPU large folio
can be first removed from deferred_split queue and swap cache, if it is there,
then the migration process begins, so that the CPU large folio will always
be out of deferred_split queue and not in swap cache. As a result, this split
function does not need to handle these two situations.
--
Best Regards,
Yan, Zi
