Re: [v2 02/11] mm/thp: zone_device awareness in THP handling code

[Balbir Singh <balbirs@nvidia.com>]

On 8/5/25 14:24, Mika Penttilä wrote:
> Hi,
> 
> On 8/5/25 07:10, Balbir Singh wrote:
>> On 8/5/25 09:26, Mika Penttilä wrote:
>>> Hi,
>>>
>>> On 8/5/25 01:46, Balbir Singh wrote:
>>>> On 8/2/25 22:13, Mika Penttilä wrote:
>>>>> Hi,
>>>>>
>>>>> On 8/2/25 13:37, Balbir Singh wrote:
>>>>>> FYI:
>>>>>>
>>>>>> I have the following patch on top of my series that seems to make it work
>>>>>> without requiring the helper to split device private folios
>>>>>>
>>>>> I think this looks much better!
>>>>>
>>>> Thanks!
>>>>
>>>>>> Signed-off-by: Balbir Singh <balbirs@nvidia.com>
>>>>>> ---
>>>>>>  include/linux/huge_mm.h |  1 -
>>>>>>  lib/test_hmm.c          | 11 +++++-
>>>>>>  mm/huge_memory.c        | 76 ++++-------------------------------------
>>>>>>  mm/migrate_device.c     | 51 +++++++++++++++++++++++++++
>>>>>>  4 files changed, 67 insertions(+), 72 deletions(-)
>>>>>>
>>>>>> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
>>>>>> index 19e7e3b7c2b7..52d8b435950b 100644
>>>>>> --- a/include/linux/huge_mm.h
>>>>>> +++ b/include/linux/huge_mm.h
>>>>>> @@ -343,7 +343,6 @@ unsigned long thp_get_unmapped_area_vmflags(struct file *filp, unsigned long add
>>>>>>  		vm_flags_t vm_flags);
>>>>>>  
>>>>>>  bool can_split_folio(struct folio *folio, int caller_pins, int *pextra_pins);
>>>>>> -int split_device_private_folio(struct folio *folio);
>>>>>>  int __split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
>>>>>>  		unsigned int new_order, bool unmapped);
>>>>>>  int min_order_for_split(struct folio *folio);
>>>>>> diff --git a/lib/test_hmm.c b/lib/test_hmm.c
>>>>>> index 341ae2af44ec..444477785882 100644
>>>>>> --- a/lib/test_hmm.c
>>>>>> +++ b/lib/test_hmm.c
>>>>>> @@ -1625,13 +1625,22 @@ static vm_fault_t dmirror_devmem_fault(struct vm_fault *vmf)
>>>>>>  	 * the mirror but here we use it to hold the page for the simulated
>>>>>>  	 * device memory and that page holds the pointer to the mirror.
>>>>>>  	 */
>>>>>> -	rpage = vmf->page->zone_device_data;
>>>>>> +	rpage = folio_page(page_folio(vmf->page), 0)->zone_device_data;
>>>>>>  	dmirror = rpage->zone_device_data;
>>>>>>  
>>>>>>  	/* FIXME demonstrate how we can adjust migrate range */
>>>>>>  	order = folio_order(page_folio(vmf->page));
>>>>>>  	nr = 1 << order;
>>>>>>  
>>>>>> +	/*
>>>>>> +	 * When folios are partially mapped, we can't rely on the folio
>>>>>> +	 * order of vmf->page as the folio might not be fully split yet
>>>>>> +	 */
>>>>>> +	if (vmf->pte) {
>>>>>> +		order = 0;
>>>>>> +		nr = 1;
>>>>>> +	}
>>>>>> +
>>>>>>  	/*
>>>>>>  	 * Consider a per-cpu cache of src and dst pfns, but with
>>>>>>  	 * large number of cpus that might not scale well.
>>>>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>>>>> index 1fc1efa219c8..863393dec1f1 100644
>>>>>> --- a/mm/huge_memory.c
>>>>>> +++ b/mm/huge_memory.c
>>>>>> @@ -72,10 +72,6 @@ static unsigned long deferred_split_count(struct shrinker *shrink,
>>>>>>  					  struct shrink_control *sc);
>>>>>>  static unsigned long deferred_split_scan(struct shrinker *shrink,
>>>>>>  					 struct shrink_control *sc);
>>>>>> -static int __split_unmapped_folio(struct folio *folio, int new_order,
>>>>>> -		struct page *split_at, struct xa_state *xas,
>>>>>> -		struct address_space *mapping, bool uniform_split);
>>>>>> -
>>>>>>  static bool split_underused_thp = true;
>>>>>>  
>>>>>>  static atomic_t huge_zero_refcount;
>>>>>> @@ -2924,51 +2920,6 @@ static void __split_huge_zero_page_pmd(struct vm_area_struct *vma,
>>>>>>  	pmd_populate(mm, pmd, pgtable);
>>>>>>  }
>>>>>>  
>>>>>> -/**
>>>>>> - * split_huge_device_private_folio - split a huge device private folio into
>>>>>> - * smaller pages (of order 0), currently used by migrate_device logic to
>>>>>> - * split folios for pages that are partially mapped
>>>>>> - *
>>>>>> - * @folio: the folio to split
>>>>>> - *
>>>>>> - * The caller has to hold the folio_lock and a reference via folio_get
>>>>>> - */
>>>>>> -int split_device_private_folio(struct folio *folio)
>>>>>> -{
>>>>>> -	struct folio *end_folio = folio_next(folio);
>>>>>> -	struct folio *new_folio;
>>>>>> -	int ret = 0;
>>>>>> -
>>>>>> -	/*
>>>>>> -	 * Split the folio now. In the case of device
>>>>>> -	 * private pages, this path is executed when
>>>>>> -	 * the pmd is split and since freeze is not true
>>>>>> -	 * it is likely the folio will be deferred_split.
>>>>>> -	 *
>>>>>> -	 * With device private pages, deferred splits of
>>>>>> -	 * folios should be handled here to prevent partial
>>>>>> -	 * unmaps from causing issues later on in migration
>>>>>> -	 * and fault handling flows.
>>>>>> -	 */
>>>>>> -	folio_ref_freeze(folio, 1 + folio_expected_ref_count(folio));
>>>>>> -	ret = __split_unmapped_folio(folio, 0, &folio->page, NULL, NULL, true);
>>>>>> -	VM_WARN_ON(ret);
>>>>>> -	for (new_folio = folio_next(folio); new_folio != end_folio;
>>>>>> -					new_folio = folio_next(new_folio)) {
>>>>>> -		zone_device_private_split_cb(folio, new_folio);
>>>>>> -		folio_ref_unfreeze(new_folio, 1 + folio_expected_ref_count(
>>>>>> -								new_folio));
>>>>>> -	}
>>>>>> -
>>>>>> -	/*
>>>>>> -	 * Mark the end of the folio split for device private THP
>>>>>> -	 * split
>>>>>> -	 */
>>>>>> -	zone_device_private_split_cb(folio, NULL);
>>>>>> -	folio_ref_unfreeze(folio, 1 + folio_expected_ref_count(folio));
>>>>>> -	return ret;
>>>>>> -}
>>>>>> -
>>>>>>  static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>>>>>>  		unsigned long haddr, bool freeze)
>>>>>>  {
>>>>>> @@ -3064,30 +3015,15 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>>>>>>  				freeze = false;
>>>>>>  			if (!freeze) {
>>>>>>  				rmap_t rmap_flags = RMAP_NONE;
>>>>>> -				unsigned long addr = haddr;
>>>>>> -				struct folio *new_folio;
>>>>>> -				struct folio *end_folio = folio_next(folio);
>>>>>>  
>>>>>>  				if (anon_exclusive)
>>>>>>  					rmap_flags |= RMAP_EXCLUSIVE;
>>>>>>  
>>>>>> -				folio_lock(folio);
>>>>>> -				folio_get(folio);
>>>>>> -
>>>>>> -				split_device_private_folio(folio);
>>>>>> -
>>>>>> -				for (new_folio = folio_next(folio);
>>>>>> -					new_folio != end_folio;
>>>>>> -					new_folio = folio_next(new_folio)) {
>>>>>> -					addr += PAGE_SIZE;
>>>>>> -					folio_unlock(new_folio);
>>>>>> -					folio_add_anon_rmap_ptes(new_folio,
>>>>>> -						&new_folio->page, 1,
>>>>>> -						vma, addr, rmap_flags);
>>>>>> -				}
>>>>>> -				folio_unlock(folio);
>>>>>> -				folio_add_anon_rmap_ptes(folio, &folio->page,
>>>>>> -						1, vma, haddr, rmap_flags);
>>>>>> +				folio_ref_add(folio, HPAGE_PMD_NR - 1);
>>>>>> +				if (anon_exclusive)
>>>>>> +					rmap_flags |= RMAP_EXCLUSIVE;
>>>>>> +				folio_add_anon_rmap_ptes(folio, page, HPAGE_PMD_NR,
>>>>>> +						 vma, haddr, rmap_flags);
>>>>>>  			}
>>>>>>  		}
>>>>>>  
>>>>>> @@ -4065,7 +4001,7 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>>>>>>  	if (nr_shmem_dropped)
>>>>>>  		shmem_uncharge(mapping->host, nr_shmem_dropped);
>>>>>>  
>>>>>> -	if (!ret && is_anon)
>>>>>> +	if (!ret && is_anon && !folio_is_device_private(folio))
>>>>>>  		remap_flags = RMP_USE_SHARED_ZEROPAGE;
>>>>>>  
>>>>>>  	remap_page(folio, 1 << order, remap_flags);
>>>>>> diff --git a/mm/migrate_device.c b/mm/migrate_device.c
>>>>>> index 49962ea19109..4264c0290d08 100644
>>>>>> --- a/mm/migrate_device.c
>>>>>> +++ b/mm/migrate_device.c
>>>>>> @@ -248,6 +248,8 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
>>>>>>  			 * page table entry. Other special swap entries are not
>>>>>>  			 * migratable, and we ignore regular swapped page.
>>>>>>  			 */
>>>>>> +			struct folio *folio;
>>>>>> +
>>>>>>  			entry = pte_to_swp_entry(pte);
>>>>>>  			if (!is_device_private_entry(entry))
>>>>>>  				goto next;
>>>>>> @@ -259,6 +261,55 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
>>>>>>  			    pgmap->owner != migrate->pgmap_owner)
>>>>>>  				goto next;
>>>>>>  
>>>>>> +			folio = page_folio(page);
>>>>>> +			if (folio_test_large(folio)) {
>>>>>> +				struct folio *new_folio;
>>>>>> +				struct folio *new_fault_folio;
>>>>>> +
>>>>>> +				/*
>>>>>> +				 * The reason for finding pmd present with a
>>>>>> +				 * device private pte and a large folio for the
>>>>>> +				 * pte is partial unmaps. Split the folio now
>>>>>> +				 * for the migration to be handled correctly
>>>>>> +				 */
>>>>>> +				pte_unmap_unlock(ptep, ptl);
>>>>>> +
>>>>>> +				folio_get(folio);
>>>>>> +				if (folio != fault_folio)
>>>>>> +					folio_lock(folio);
>>>>>> +				if (split_folio(folio)) {
>>>>>> +					if (folio != fault_folio)
>>>>>> +						folio_unlock(folio);
>>>>>> +					ptep = pte_offset_map_lock(mm, pmdp, addr, &ptl);
>>>>>> +					goto next;
>>>>>> +				}
>>>>>> +
>>>>> The nouveau migrate_to_ram handler needs adjustment also if split happens.
>>>>>
>>>> test_hmm needs adjustment because of the way the backup folios are setup.
>>> nouveau should check the folio order after the possible split happens.
>>>
>> You mean the folio_split callback?
> 
> no, nouveau_dmem_migrate_to_ram():
> ..
>         sfolio = page_folio(vmf->page);
>         order = folio_order(sfolio);
> ...
>         migrate_vma_setup()
> ..
> if sfolio is split order still reflects the pre-split order
> 

Will fix, good catch!

>>
>>>>>> +				/*
>>>>>> +				 * After the split, get back the extra reference
>>>>>> +				 * on the fault_page, this reference is checked during
>>>>>> +				 * folio_migrate_mapping()
>>>>>> +				 */
>>>>>> +				if (migrate->fault_page) {
>>>>>> +					new_fault_folio = page_folio(migrate->fault_page);
>>>>>> +					folio_get(new_fault_folio);
>>>>>> +				}
>>>>>> +
>>>>>> +				new_folio = page_folio(page);
>>>>>> +				pfn = page_to_pfn(page);
>>>>>> +
>>>>>> +				/*
>>>>>> +				 * Ensure the lock is held on the correct
>>>>>> +				 * folio after the split
>>>>>> +				 */
>>>>>> +				if (folio != new_folio) {
>>>>>> +					folio_unlock(folio);
>>>>>> +					folio_lock(new_folio);
>>>>>> +				}
>>>>> Maybe careful not to unlock fault_page ?
>>>>>
>>>> split_page will unlock everything but the original folio, the code takes the lock
>>>> on the folio corresponding to the new folio
>>> I mean do_swap_page() unlocks folio of fault_page and expects it to remain locked.
>>>
>> Not sure I follow what you're trying to elaborate on here
> 
> do_swap_page:
> ..
>         if (trylock_page(vmf->page)) {
>                 ret = pgmap->ops->migrate_to_ram(vmf);
>                                <- vmf->page should be locked here even after split
>                 unlock_page(vmf->page);
> 

Yep, the split will unlock all tail folios, leaving the just head folio locked
and this the change, the lock we need to hold is the folio lock associated with
fault_page, pte entry and not unlock when the cause is a fault. The code seems
to do the right thing there, let me double check

Balbir
and the code does the right thing there.