Message-ID: <aHnAgjaJeyKM+Osm@lstrano-desk.jf.intel.com>
Date: Thu, 17 Jul 2025 20:33:22 -0700
From: Matthew Brost <matthew.brost@...el.com>
To: Zi Yan <ziy@...dia.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 Thu, Jul 17, 2025 at 09:25:02PM -0400, Zi Yan wrote:
> 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
> 

Will take a look. It is possible some of the issues we are hitting are
due to working on drm-tip + pulling in core MM patches in this series on
top of that branch then missing some other patches in mm-new. I'll see
if ww can figure out a work flow to have the latest and greatest from
both drm-tip and the MM branches.

Will these changes be in 6.17?

> 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.
>

Thanks.

Matt
 
> Best Regards,
> Yan, Zi

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