Re: [v1 resend 08/12] mm/thp: add split during migration support

[Zi Yan <ziy@nvidia.com>]

On 16 Jul 2025, at 1:34, Matthew Brost wrote:

> On Sun, Jul 06, 2025 at 11:47:10AM +1000, Balbir Singh wrote:
>> On 7/6/25 11:34, Zi Yan wrote:
>>> On 5 Jul 2025, at 21:15, Balbir Singh wrote:
>>>
>>>> On 7/5/25 11:55, Zi Yan wrote:
>>>>> On 4 Jul 2025, at 20:58, Balbir Singh wrote:
>>>>>
>>>>>> On 7/4/25 21:24, Zi Yan wrote:
>>>>>>>
>>>>>>> s/pages/folio
>>>>>>>
>>>>>>
>>>>>> Thanks, will make the changes
>>>>>>
>>>>>>> Why name it isolated if the folio is unmapped? Isolated folios often mean
>>>>>>> they are removed from LRU lists. isolated here causes confusion.
>>>>>>>
>>>>>>
>>>>>> Ack, will change the name
>>>>>>
>>>>>>
>>>>>>>>   *
>>>>>>>>   * 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
>>>>>>>> @@ -3800,7 +3799,7 @@ 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)
>>>>>>>> +		struct list_head *list, bool uniform_split, bool isolated)
>>>>>>>>  {
>>>>>>>>  	struct deferred_split *ds_queue = get_deferred_split_queue(folio);
>>>>>>>>  	XA_STATE(xas, &folio->mapping->i_pages, folio->index);
>>>>>>>> @@ -3846,14 +3845,16 @@ 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 (!isolated) {
>>>>>>>> +			anon_vma = folio_get_anon_vma(folio);
>>>>>>>> +			if (!anon_vma) {
>>>>>>>> +				ret = -EBUSY;
>>>>>>>> +				goto out;
>>>>>>>> +			}
>>>>>>>> +			anon_vma_lock_write(anon_vma);
>>>>>>>>  		}
>>>>>>>>  		end = -1;
>>>>>>>>  		mapping = NULL;
>>>>>>>> -		anon_vma_lock_write(anon_vma);
>>>>>>>>  	} else {
>>>>>>>>  		unsigned int min_order;
>>>>>>>>  		gfp_t gfp;
>>>>>>>> @@ -3920,7 +3921,8 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>>>>>>>>  		goto out_unlock;
>>>>>>>>  	}
>>>>>>>>
>>>>>>>> -	unmap_folio(folio);
>>>>>>>> +	if (!isolated)
>>>>>>>> +		unmap_folio(folio);
>>>>>>>>
>>>>>>>>  	/* block interrupt reentry in xa_lock and spinlock */
>>>>>>>>  	local_irq_disable();
>>>>>>>> @@ -3973,14 +3975,15 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>>>>>>>>
>>>>>>>>  		ret = __split_unmapped_folio(folio, new_order,
>>>>>>>>  				split_at, lock_at, list, end, &xas, mapping,
>>>>>>>> -				uniform_split);
>>>>>>>> +				uniform_split, isolated);
>>>>>>>>  	} else {
>>>>>>>>  		spin_unlock(&ds_queue->split_queue_lock);
>>>>>>>>  fail:
>>>>>>>>  		if (mapping)
>>>>>>>>  			xas_unlock(&xas);
>>>>>>>>  		local_irq_enable();
>>>>>>>> -		remap_page(folio, folio_nr_pages(folio), 0);
>>>>>>>> +		if (!isolated)
>>>>>>>> +			remap_page(folio, folio_nr_pages(folio), 0);
>>>>>>>>  		ret = -EAGAIN;
>>>>>>>>  	}
>>>>>>>
>>>>>>> These "isolated" special handlings does not look good, I wonder if there
>>>>>>> is a way of letting split code handle device private folios more gracefully.
>>>>>>> It also causes confusions, since why does "isolated/unmapped" folios
>>>>>>> not need to unmap_page(), remap_page(), or unlock?
>>>>>>>
>>>>>>>
>>>>>>
>>>>>> There are two reasons for going down the current code path
>>>>>
>>>>> After thinking more, I think adding isolated/unmapped is not the right
>>>>> way, since unmapped folio is a very generic concept. If you add it,
>>>>> one can easily misuse the folio split code by first unmapping a folio
>>>>> and trying to split it with unmapped = true. I do not think that is
>>>>> supported and your patch does not prevent that from happening in the future.
>>>>>
>>>>
>>>> I don't understand the misuse case you mention, I assume you mean someone can
>>>> get the usage wrong? The responsibility is on the caller to do the right thing
>>>> if calling the API with unmapped
>>>
>>> Before your patch, there is no use case of splitting unmapped folios.
>>> Your patch only adds support for device private page split, not any unmapped
>>> folio split. So using a generic isolated/unmapped parameter is not OK.
>>>
>>
>> There is a use for splitting unmapped folios (see below)
>>
>>>>
>>>>> You should teach different parts of folio split code path to handle
>>>>> device private folios properly. Details are below.
>>>>>
>>>>>>
>>>>>> 1. if the isolated check is not present, folio_get_anon_vma will fail and cause
>>>>>>    the split routine to return with -EBUSY
>>>>>
>>>>> You do something below instead.
>>>>>
>>>>> if (!anon_vma && !folio_is_device_private(folio)) {
>>>>> 	ret = -EBUSY;
>>>>> 	goto out;
>>>>> } else if (anon_vma) {
>>>>> 	anon_vma_lock_write(anon_vma);
>>>>> }
>>>>>
>>>>
>>>> folio_get_anon() cannot be called for unmapped folios. In our case the page has
>>>> already been unmapped. Is there a reason why you mix anon_vma_lock_write with
>>>> the check for device private folios?
>>>
>>> Oh, I did not notice that anon_vma = folio_get_anon_vma(folio) is also
>>> in if (!isolated) branch. In that case, just do
>>>
>>> if (folio_is_device_private(folio) {
>>> ...
>>> } else if (is_anon) {
>>> ...
>>> } else {
>>> ...
>>> }
>>>
>>>>
>>>>> People can know device private folio split needs a special handling.
>>>>>
>>>>> BTW, why a device private folio can also be anonymous? Does it mean
>>>>> if a page cache folio is migrated to device private, kernel also
>>>>> sees it as both device private and file-backed?
>>>>>
>>>>
>>>> FYI: device private folios only work with anonymous private pages, hence
>>>> the name device private.
>>>
>>> OK.
>>>
>>>>
>>>>>
>>>>>> 2. Going through unmap_page(), remap_page() causes a full page table walk, which
>>>>>>    the migrate_device API has already just done as a part of the migration. The
>>>>>>    entries under consideration are already migration entries in this case.
>>>>>>    This is wasteful and in some case unexpected.
>>>>>
>>>>> unmap_folio() already adds TTU_SPLIT_HUGE_PMD to try to split
>>>>> PMD mapping, which you did in migrate_vma_split_pages(). You probably
>>>>> can teach either try_to_migrate() or try_to_unmap() to just split
>>>>> device private PMD mapping. Or if that is not preferred,
>>>>> you can simply call split_huge_pmd_address() when unmap_folio()
>>>>> sees a device private folio.
>>>>>
>>>>> For remap_page(), you can simply return for device private folios
>>>>> like it is currently doing for non anonymous folios.
>>>>>
>>>>
>>>> Doing a full rmap walk does not make sense with unmap_folio() and
>>>> remap_folio(), because
>>>>
>>>> 1. We need to do a page table walk/rmap walk again
>>>> 2. We'll need special handling of migration <-> migration entries
>>>>    in the rmap handling (set/remove migration ptes)
>>>> 3. In this context, the code is already in the middle of migration,
>>>>    so trying to do that again does not make sense.
>>>
>>> Why doing split in the middle of migration? Existing split code
>>> assumes to-be-split folios are mapped.
>>>
>>> What prevents doing split before migration?
>>>
>>
>> The code does do a split prior to migration if THP selection fails
>>
>> Please see https://lore.kernel.org/lkml/20250703233511.2028395-5-balbirs@nvidia.com/
>> and the fallback part which calls split_folio()
>>
>> But the case under consideration is special since the device needs to allocate
>> corresponding pfn's as well. The changelog mentions it:
>>
>> "The common case that arises is that after setup, during migrate
>> the destination might not be able to allocate MIGRATE_PFN_COMPOUND
>> pages."
>>
>> I can expand on it, because migrate_vma() is a multi-phase operation
>>
>> 1. migrate_vma_setup()
>> 2. migrate_vma_pages()
>> 3. migrate_vma_finalize()
>>
>> It can so happen that when we get the destination pfn's allocated the destination
>> might not be able to allocate a large page, so we do the split in migrate_vma_pages().
>>
>> The pages have been unmapped and collected in migrate_vma_setup()
>>
>> The next patch in the series 9/12 (https://lore.kernel.org/lkml/20250703233511.2028395-10-balbirs@nvidia.com/)
>> tests the split and emulates a failure on the device side to allocate large pages
>> and tests it in 10/12 (https://lore.kernel.org/lkml/20250703233511.2028395-11-balbirs@nvidia.com/)
>>
>
> Another use case I’ve seen is when a previously allocated high-order
> folio, now in the free memory pool, is reallocated as a lower-order
> page. For example, a 2MB fault allocates a folio, the memory is later

That is different. If the high-order folio is free, it should be split
using split_page() from mm/page_alloc.c.

> freed, and then a 4KB fault reuses a page from that previously allocated
> folio. This will be actually quite common in Xe / GPU SVM. In such
> cases, the folio in an unmapped state needs to be split. I’d suggest a

This folio is unused, so ->flags, ->mapping, and etc. are not set,
__split_unmapped_folio() is not for it, unless you mean free folio
differently.

> migrate_device_* helper built on top of the core MM __split_folio
> function add here.
>

--
Best Regards,
Yan, Zi