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

[Balbir Singh <balbirs@nvidia.com>]

On 9/25/25 19:43, David Hildenbrand wrote:
> On 25.09.25 01:58, 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.
> 
> I think one reason might be that in contrast to ordinary pages, zone-device memory is only ever used to be used for folios, right?
> 
> Would there be a user that just allocates pages and not wants a folio associated with it?
> 

A non-THP aware driver use case would be a potential use case for zero order folios (also pages at the moment). 

> It's a good question of that would look like when we have dynamically allocated struct folio ...

I think for dynamically allocated folios we could probably do away with pages, but not 100% sure at the moment.

> 
>>
>>>>
>>>> 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()?
> 
> Yeah, that's also what I thought.
> 

Balbir