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

[Balbir Singh <balbirs@nvidia.com>]

On 9/25/25 09:58, Alistair Popple wrote:
> On 2025-09-25 at 03:36 +1000, Zi Yan <ziy@nvidia.com> wrote...
>> On 24 Sep 2025, at 6:55, David Hildenbrand wrote:
>>
>>> 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?
>>
>> Right. Either we would waste memory by simply concatenating all “struct folio”
>> and putting paddings at the end, or we would free tail “struct folio” first,
>> then allocate tail “struct page”. Both are painful and do not match core mm’s
>> memdesc pattern, where “struct folio” is allocated when caller is asking
>> for a folio. If “struct folio” is always allocated, there is no difference
>> between “struct folio” and “struct page”.
> 
> As mentioned in my other reply I need to investigate this some more, but I
> don't think we _need_ to always allocate folios (or pages for that matter).
> The ZONE_DEVICE code just uses folios/pages for interacting with the core mm,
> not for managing the device memory itself, so we should be able to make it more
> closely match the memdesc pattern. It's just I'm still a bit unsure what that
> pattern will actually look like.
> 
>>>
>>> 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.
> 
> 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().
> 
> Where would the order parameter come from? Presumably
> folio_order(compound_head(page)) in which case shouldn't the op actually just be
> pgmap->ops->folio_free()?
> 
->page_free() can detect if the page is of large order. The patchset was designed
to make folios and opt-in and avoid unnecessary changes to existing drivers.
But I can revisit that thought process if it helps with cleaner code.

Balbir