Re: [v7 11/16] mm/migrate_device: add THP splitting during migration

[Zi Yan <ziy@nvidia.com>]

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