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

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

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

Matt

> 
> >>
> >>
> >>>
> >>> For lru_add_split_folio(), you can skip it if a device private
> >>> folio is seen.
> >>>
> >>> Last, for unlock part, why do you need to keep all after-split folios
> >>> locked? It should be possible to just keep the to-be-migrated folio
> >>> locked and unlock the rest for a later retry. But I could miss something
> >>> since I am not familiar with device private migration code.
> >>>
> >>
> >> Not sure I follow this comment
> > 
> > Because the patch is doing split in the middle of migration and existing
> > split code never supports. My comment is based on the assumption that
> > the split is done when a folio is mapped.
> > 
> 
> Understood, hopefully I've explained the reason for the split in the middle
> of migration
> 
> Thanks for the detailed review
> Balbir