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

[Matthew Brost <matthew.brost@intel.com>]

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.

Matt 

> 
> Best Regards,
> Yan, Zi