Re: [v6 01/15] mm/zone_device: support large zone device private folios

[David Hildenbrand <david@redhat.com>]

On 18.09.25 04:49, Zi Yan wrote:
> On 16 Sep 2025, at 8:21, Balbir Singh wrote:
> 
>> Add routines to support allocation of large order zone device folios
>> and helper functions for zone device folios, to check if a folio is
>> device private and helpers for setting zone device data.
>>
>> When large folios are used, the existing page_free() callback in
>> pgmap is called when the folio is freed, this is true for both
>> PAGE_SIZE and higher order pages.
>>
>> Zone device private large folios do not support deferred split and
>> scan like normal THP folios.
>>
>> Signed-off-by: Balbir Singh <balbirs@nvidia.com>
>> Cc: David Hildenbrand <david@redhat.com>
>> Cc: Zi Yan <ziy@nvidia.com>
>> Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
>> Cc: Rakie Kim <rakie.kim@sk.com>
>> Cc: Byungchul Park <byungchul@sk.com>
>> Cc: Gregory Price <gourry@gourry.net>
>> Cc: Ying Huang <ying.huang@linux.alibaba.com>
>> Cc: Alistair Popple <apopple@nvidia.com>
>> Cc: Oscar Salvador <osalvador@suse.de>
>> Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
>> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
>> Cc: "Liam R. Howlett" <Liam.Howlett@oracle.com>
>> Cc: Nico Pache <npache@redhat.com>
>> Cc: Ryan Roberts <ryan.roberts@arm.com>
>> Cc: Dev Jain <dev.jain@arm.com>
>> Cc: Barry Song <baohua@kernel.org>
>> Cc: Lyude Paul <lyude@redhat.com>
>> Cc: Danilo Krummrich <dakr@kernel.org>
>> Cc: David Airlie <airlied@gmail.com>
>> Cc: Simona Vetter <simona@ffwll.ch>
>> Cc: Ralph Campbell <rcampbell@nvidia.com>
>> Cc: Mika Penttilä <mpenttil@redhat.com>
>> Cc: Matthew Brost <matthew.brost@intel.com>
>> Cc: Francois Dugast <francois.dugast@intel.com>
>> ---
>>   include/linux/memremap.h | 10 +++++++++-
>>   mm/memremap.c            | 34 +++++++++++++++++++++-------------
>>   mm/rmap.c                |  6 +++++-
>>   3 files changed, 35 insertions(+), 15 deletions(-)
>>
>> diff --git a/include/linux/memremap.h b/include/linux/memremap.h
>> index e5951ba12a28..9c20327c2be5 100644
>> --- a/include/linux/memremap.h
>> +++ b/include/linux/memremap.h
>> @@ -206,7 +206,7 @@ static inline bool is_fsdax_page(const struct page *page)
>>   }
>>
>>   #ifdef CONFIG_ZONE_DEVICE
>> -void zone_device_page_init(struct page *page);
>> +void zone_device_folio_init(struct folio *folio, unsigned int order);
>>   void *memremap_pages(struct dev_pagemap *pgmap, int nid);
>>   void memunmap_pages(struct dev_pagemap *pgmap);
>>   void *devm_memremap_pages(struct device *dev, struct dev_pagemap *pgmap);
>> @@ -215,6 +215,14 @@ struct dev_pagemap *get_dev_pagemap(unsigned long pfn);
>>   bool pgmap_pfn_valid(struct dev_pagemap *pgmap, unsigned long pfn);
>>
>>   unsigned long memremap_compat_align(void);
>> +
>> +static inline void zone_device_page_init(struct page *page)
>> +{
>> +	struct folio *folio = page_folio(page);
>> +
>> +	zone_device_folio_init(folio, 0);
> 
> I assume it is for legacy code, where only non-compound page exists?
> 
> It seems that you assume @page is always order-0, but there is no check
> for it. Adding VM_WARN_ON_ONCE_FOLIO(folio_order(folio) != 0, folio)
> above it would be useful to detect misuse.
> 
>> +}
>> +
>>   #else
>>   static inline void *devm_memremap_pages(struct device *dev,
>>   		struct dev_pagemap *pgmap)
>> diff --git a/mm/memremap.c b/mm/memremap.c
>> index 46cb1b0b6f72..a8481ebf94cc 100644
>> --- a/mm/memremap.c
>> +++ b/mm/memremap.c
>> @@ -416,20 +416,19 @@ EXPORT_SYMBOL_GPL(get_dev_pagemap);
>>   void free_zone_device_folio(struct folio *folio)
>>   {
>>   	struct dev_pagemap *pgmap = folio->pgmap;
>> +	unsigned long nr = folio_nr_pages(folio);
>> +	int i;
>>
>>   	if (WARN_ON_ONCE(!pgmap))
>>   		return;
>>
>>   	mem_cgroup_uncharge(folio);
>>
>> -	/*
>> -	 * Note: we don't expect anonymous compound pages yet. Once supported
>> -	 * and we could PTE-map them similar to THP, we'd have to clear
>> -	 * PG_anon_exclusive on all tail pages.
>> -	 */
>>   	if (folio_test_anon(folio)) {
>> -		VM_BUG_ON_FOLIO(folio_test_large(folio), folio);
>> -		__ClearPageAnonExclusive(folio_page(folio, 0));
>> +		for (i = 0; i < nr; i++)
>> +			__ClearPageAnonExclusive(folio_page(folio, i));
>> +	} else {
>> +		VM_WARN_ON_ONCE(folio_test_large(folio));
>>   	}
>>
>>   	/*
>> @@ -456,8 +455,8 @@ void free_zone_device_folio(struct folio *folio)
>>   	case MEMORY_DEVICE_COHERENT:
>>   		if (WARN_ON_ONCE(!pgmap->ops || !pgmap->ops->page_free))
>>   			break;
>> -		pgmap->ops->page_free(folio_page(folio, 0));
>> -		put_dev_pagemap(pgmap);
>> +		pgmap->ops->page_free(&folio->page);
>> +		percpu_ref_put_many(&folio->pgmap->ref, nr);
>>   		break;
>>
>>   	case MEMORY_DEVICE_GENERIC:
>> @@ -480,14 +479,23 @@ void free_zone_device_folio(struct folio *folio)
>>   	}
>>   }
>>
>> -void zone_device_page_init(struct page *page)
>> +void zone_device_folio_init(struct folio *folio, unsigned int order)
>>   {
>> +	struct page *page = folio_page(folio, 0);
> 
> It is strange to see a folio is converted back to page in
> a function called zone_device_folio_init().
> 
>> +
>> +	VM_WARN_ON_ONCE(order > MAX_ORDER_NR_PAGES);
>> +
>>   	/*
>>   	 * Drivers shouldn't be allocating pages after calling
>>   	 * memunmap_pages().
>>   	 */
>> -	WARN_ON_ONCE(!percpu_ref_tryget_live(&page_pgmap(page)->ref));
>> -	set_page_count(page, 1);
>> +	WARN_ON_ONCE(!percpu_ref_tryget_many(&page_pgmap(page)->ref, 1 << order));
>> +	folio_set_count(folio, 1);
>>   	lock_page(page);
>> +
>> +	if (order > 1) {
>> +		prep_compound_page(page, order);
>> +		folio_set_large_rmappable(folio);
>> +	}
> 
> OK, so basically, @folio is not a compound page yet when zone_device_folio_init()
> is called.
> 
> I feel that your zone_device_page_init() and zone_device_folio_init()
> implementations are inverse. They should follow the same pattern
> as __alloc_pages_noprof() and __folio_alloc_noprof(), where
> zone_device_page_init() does the actual initialization and
> zone_device_folio_init() just convert a page to folio.
> 
> Something like:
> 
> void zone_device_page_init(struct page *page, unsigned int order)
> {
> 	VM_WARN_ON_ONCE(order > MAX_ORDER_NR_PAGES);
> 
> 	/*
> 	 * Drivers shouldn't be allocating pages after calling
> 	 * memunmap_pages().
> 	 */
> 
>      WARN_ON_ONCE(!percpu_ref_tryget_many(&page_pgmap(page)->ref, 1 << order));
> 	
> 	/*
> 	 * anonymous folio does not support order-1, high order file-backed folio
> 	 * is not supported at all.
> 	 */
> 	VM_WARN_ON_ONCE(order == 1);
> 
> 	if (order > 1)
> 		prep_compound_page(page, order);
> 
> 	/* page has to be compound head here */
> 	set_page_count(page, 1);
> 	lock_page(page);
> }
> 
> void zone_device_folio_init(struct folio *folio, unsigned int order)
> {
> 	struct page *page = folio_page(folio, 0);
> 
> 	zone_device_page_init(page, order);
> 	page_rmappable_folio(page);
> }
> 
> Or
> 
> struct folio *zone_device_folio_init(struct page *page, unsigned int order)
> {
> 	zone_device_page_init(page, order);
> 	return page_rmappable_folio(page);
> }

I think the problem is that it will all be weird once we dynamically 
allocate "struct folio".

I have not yet a clear understanding on how that would really work.

For example, should it be pgmap->ops->page_folio() ?

Who allocates the folio? Do we allocate all order-0 folios initially, to 
then merge them when constructing large folios? How do we manage the 
"struct folio" during such merging splitting?

With that in mind, I don't really know what the proper interface should 
be today.


zone_device_folio_init(struct page *page, unsigned int order)

looks cleaner, agreed.

> 
> 
> Then, it comes to free_zone_device_folio() above,
> I feel that pgmap->ops->page_free() should take an additional order
> parameter to free a compound page like free_frozen_pages().
> 

IIRC free_frozen_pages() does not operate on compound pages. If we know 
that we are operating on a compound page (or single page) then passing 
in the page (or better the folio) should work.

-- 
Cheers

David / dhildenb