Re: [v2 01/11] mm/zone_device: support large zone device private folios

[Balbir Singh <balbirs@nvidia.com>]

On 7/30/25 19:50, David Hildenbrand wrote:
> On 30.07.25 11:21, 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.
>>
>> Cc: Karol Herbst <kherbst@redhat.com>
>> 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: "Jérôme Glisse" <jglisse@redhat.com>
>> Cc: Shuah Khan <shuah@kernel.org>
>> Cc: David Hildenbrand <david@redhat.com>
>> Cc: Barry Song <baohua@kernel.org>
>> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
>> Cc: Ryan Roberts <ryan.roberts@arm.com>
>> Cc: Matthew Wilcox <willy@infradead.org>
>> Cc: Peter Xu <peterx@redhat.com>
>> Cc: Zi Yan <ziy@nvidia.com>
>> Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
>> Cc: Jane Chu <jane.chu@oracle.com>
>> Cc: Alistair Popple <apopple@nvidia.com>
>> Cc: Donet Tom <donettom@linux.ibm.com>
>> 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>
>>
>> Signed-off-by: Balbir Singh <balbirs@nvidia.com>
>> ---
>>   include/linux/memremap.h | 10 ++++++++-
>>   mm/memremap.c            | 48 +++++++++++++++++++++++++++++-----------
>>   2 files changed, 44 insertions(+), 14 deletions(-)
>>
>> diff --git a/include/linux/memremap.h b/include/linux/memremap.h
>> index 4aa151914eab..a0723b35eeaa 100644
>> --- a/include/linux/memremap.h
>> +++ b/include/linux/memremap.h
>> @@ -199,7 +199,7 @@ static inline bool folio_is_fsdax(const struct folio *folio)
>>   }
>>     #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);
>> @@ -209,6 +209,14 @@ 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);
>> +}
>> +
>>   #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 b0ce0d8254bd..3ca136e7455e 100644
>> --- a/mm/memremap.c
>> +++ b/mm/memremap.c
>> @@ -427,20 +427,19 @@ EXPORT_SYMBOL_GPL(get_dev_pagemap);
>>   void free_zone_device_folio(struct folio *folio)
>>   {
>>       struct dev_pagemap *pgmap = folio->pgmap;
>> +    unsigned int nr = folio_nr_pages(folio);
>> +    int i;
> 
> "unsigned long" is to be future-proof.

Will change this for v3

> 
> (folio_nr_pages() returns long and probably soon unsigned long)
> 
> [ I'd probably all it "nr_pages" ]

Ack

> 
>>         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));
>>       }
>>         /*
>> @@ -464,11 +463,20 @@ void free_zone_device_folio(struct folio *folio)
>>         switch (pgmap->type) {
>>       case MEMORY_DEVICE_PRIVATE:
>> +        if (folio_test_large(folio)) {
> 
> Could do "nr > 1" if we already have that value around.
> 

Ack

>> +            folio_unqueue_deferred_split(folio);
> 
> I think I asked that already but maybe missed the reply: Should these folios ever be added to the deferred split queue and is there any value in splitting them under memory pressure in the shrinker?
> 
> My gut feeling is "No", because the buddy cannot make use of these folios, but maybe there is an interesting case where we want that behavior?
> 
>> +
>> +            percpu_ref_put_many(&folio->pgmap->ref, nr - 1);
>> +        }
>> +        pgmap->ops->page_free(&folio->page);
>> +        percpu_ref_put(&folio->pgmap->ref);
> 
> Coold you simply do a
> 
>     percpu_ref_put_many(&folio->pgmap->ref, nr);
> 
> here, or would that be problematic?
> 

I can definitely try that

>> +        folio->page.mapping = NULL;
>> +        break;
>>       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(&folio->pgmap->ref);
>>           break;
>>         case MEMORY_DEVICE_GENERIC:
>> @@ -491,14 +499,28 @@ 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);
>> +
>> +    VM_WARN_ON_ONCE(order > MAX_ORDER_NR_PAGES);
>> +
>> +    /*
>> +     * Only PMD level migration is supported for THP migration
>> +     */
> 
> Talking about something that does not exist yet (and is very specific) sounds a bit weird.
> 
> Should this go into a different patch, or could we rephrase the comment to be a bit more generic?
> 
> In this patch here, nothing would really object to "order" being intermediate.
> 
> (also, this is a device_private limitation? shouldn't that check go somehwere where we can perform this device-private limitation check?)
> 

I can remove the limitation and keep it generic

>> +    WARN_ON_ONCE(order && order != HPAGE_PMD_ORDER);
>> +
>>       /*
>>        * 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);
>> +    }
>>   }
>> -EXPORT_SYMBOL_GPL(zone_device_page_init);
>> +EXPORT_SYMBOL_GPL(zone_device_folio_init);
> 
> 

Thanks,
Balbir