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

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

On Thu, Jul 17, 2025 at 07:53:40AM +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.

> >>> 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.

Matt

> Balbir