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

[Balbir Singh <balbirs@nvidia.com>]

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?

>>
>>>> +				/*
>>>> +				 * 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

Balbir