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

[Zi Yan <ziy@nvidia.com>]

On 17 Jul 2025, at 18:24, Matthew Brost wrote:

> On Thu, Jul 17, 2025 at 07:53:40AM +1000, Balbir Singh wrote:
>> On 7/17/25 02:24, Matthew Brost wrote:
>>> On Wed, Jul 16, 2025 at 07:19:10AM -0400, Zi Yan wrote:
>>>> 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.
>>>>
>>>
>>> Ah, ok. Let me see if that works - it would easier.
>>>
>
> This suggestion quickly blows up as PageCompound is true and page_count
> here is zero.

OK, your folio has PageCompound set. Then you will need __split_unmapped_foio().

>
>>>>> 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.
>>>>
>>>
>>> This is right, those fields should be clear.
>>>
>>> Thanks for the tip.
>>>
>> I was hoping to reuse __split_folio_to_order() at some point in the future
>> to split the backing pages in the driver, but it is not an immediate priority
>>
>
> I think we need something for the scenario I describe here. I was to
> make __split_huge_page_to_list_to_order with a couple of hacks but it
> almostly certainig not right as Zi pointed out.
>
> New to the MM stuff, but play around with this a bit and see if I can
> come up with something that will work here.

Can you try to write a new split_page function with __split_unmapped_folio()?
Since based on your description, your folio is not mapped.


Best Regards,
Yan, Zi