Message-ID: <9F4425D6-609F-40C8-BF24-2455F15234A7@nvidia.com>
Date: Thu, 17 Jul 2025 21:25:02 -0400
From: Zi Yan <ziy@...dia.com>
To: Matthew Brost <matthew.brost@...el.com>
Cc: Balbir Singh <balbirs@...dia.com>, linux-mm@...ck.org,
 akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
 Karol Herbst <kherbst@...hat.com>, Lyude Paul <lyude@...hat.com>,
 Danilo Krummrich <dakr@...nel.org>, David Airlie <airlied@...il.com>,
 Simona Vetter <simona@...ll.ch>,
 Jérôme Glisse <jglisse@...hat.com>,
 Shuah Khan <shuah@...nel.org>, David Hildenbrand <david@...hat.com>,
 Barry Song <baohua@...nel.org>, Baolin Wang <baolin.wang@...ux.alibaba.com>,
 Ryan Roberts <ryan.roberts@....com>, Matthew Wilcox <willy@...radead.org>,
 Peter Xu <peterx@...hat.com>, Kefeng Wang <wangkefeng.wang@...wei.com>,
 Jane Chu <jane.chu@...cle.com>, Alistair Popple <apopple@...dia.com>,
 Donet Tom <donettom@...ux.ibm.com>
Subject: Re: [v1 resend 08/12] mm/thp: add split during migration support

On 17 Jul 2025, at 20:41, Matthew Brost wrote:

> On Thu, Jul 17, 2025 at 07:04:48PM -0400, Zi Yan wrote:
>> 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.
>>
>
> Yes, page->mapping is NULL in this case - that was part of the hacks to
> __split_huge_page_to_list_to_order (more specially __folio_split) I had
> to make in order to get something working for this case.
>
> I can try out something based on __split_unmapped_folio and report back.

mm-new tree has an updated __split_unmapped_folio() version, it moves
all unmap irrelevant code out of __split_unmaped_folio(). You might find
it easier to reuse.

See: https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git/tree/mm/huge_memory.c?h=mm-new#n3430

I am about to update the code with v4 patches. I will cc you, so that
you can get the updated __split_unmaped_folio().

Feel free to ask questions on folio split code.

Best Regards,
Yan, Zi

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