[RFC 01/11] mm/zone_device: support large zone device private folios
Balbir Singh
balbirs at nvidia.com
Wed Jul 9 05:25:26 UTC 2025
On 7/8/25 23:37, David Hildenbrand wrote:
> On 06.03.25 05:42, 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.
>>
>> Signed-off-by: Balbir Singh <balbirs at nvidia.com>
>> ---
>> include/linux/memremap.h | 22 +++++++++++++++++-
>> mm/memremap.c | 50 +++++++++++++++++++++++++++++-----------
>> 2 files changed, 58 insertions(+), 14 deletions(-)
>>
>> diff --git a/include/linux/memremap.h b/include/linux/memremap.h
>> index 4aa151914eab..11d586dd8ef1 100644
>> --- a/include/linux/memremap.h
>> +++ b/include/linux/memremap.h
>> @@ -169,6 +169,18 @@ static inline bool folio_is_device_private(const struct folio *folio)
>> return is_device_private_page(&folio->page);
>> }
>> +static inline void *folio_zone_device_data(const struct folio *folio)
>> +{
>> + VM_BUG_ON_FOLIO(!folio_is_device_private(folio), folio);
>> + return folio->page.zone_device_data;
>> +}
>
> Not used.
>
>> +
>> +static inline void folio_set_zone_device_data(struct folio *folio, void *data)
>> +{
>> + VM_BUG_ON_FOLIO(!folio_is_device_private(folio), folio);
>> + folio->page.zone_device_data = data;
>> +}
>> +
>
> Not used.
>
> Move both into the patch where they are actually used.
>
Ack
>> static inline bool is_pci_p2pdma_page(const struct page *page)
>> {
>> return IS_ENABLED(CONFIG_PCI_P2PDMA) &&
>> @@ -199,7 +211,7 @@ static inline bool folio_is_fsdax(const struct folio *folio)
>> }
>> #ifdef CONFIG_ZONE_DEVICE
>> -void zone_device_page_init(struct page *page);
>> +void init_zone_device_folio(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 +221,14 @@ 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);
>> +
>> + init_zone_device_folio(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 2aebc1b192da..7d98d0a4c0cd 100644
>> --- a/mm/memremap.c
>> +++ b/mm/memremap.c
>> @@ -459,20 +459,21 @@ 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;
>> + bool anon = folio_test_anon(folio);
>
> You can easily get rid of this (see below).
>
>> + struct page *page = folio_page(folio, 0);
>
> Please inline folio_page(folio, 0) below instead.
Sure, is that preferred to taking a struct page ref?
>
>> 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));
>> + WARN_ON_ONCE(folio_test_large(folio) && !anon);
>> +
>> + for (i = 0; i < nr; i++) {
>> + if (anon)
>> + __ClearPageAnonExclusive(folio_page(folio, i));
>> }
>
> if (folio_test_anon(folio)) {
> for (i = 0; i < nr; i++)
> __ClearPageAnonExclusive(folio_page(folio, i));
> } else {
> VM_WARN_ON_ONCE(folio_test_large(folio));
> }
>
Ack
>> /*
>> @@ -496,10 +497,19 @@ void free_zone_device_folio(struct folio *folio)
>> switch (pgmap->type) {
>> case MEMORY_DEVICE_PRIVATE:
>> + if (folio_test_large(folio)) {
>> + folio_unqueue_deferred_split(folio);
>
> Is deferred splitting even a thing for device-private?
>
> Should we ever queue them for deferred splitting?
>
Not really, but wanted to do the right thing in the tear down path, I can remove these bits
>> +
>> + percpu_ref_put_many(&folio->pgmap->ref, nr - 1);
>
> Looks like we instead want a helper put_dev_pagemap_refs(pgmap, nr) below instead
>
>> + }
>> + pgmap->ops->page_free(page);
>> + put_dev_pagemap(pgmap);
>> + 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));
>> + pgmap->ops->page_free(page);
>> put_dev_pagemap(pgmap);
>> break;
>> @@ -523,14 +533,28 @@ void free_zone_device_folio(struct folio *folio)
>> }
>> }
>> -void zone_device_page_init(struct page *page)
>> +void init_zone_device_folio(struct folio *folio, unsigned int order)
>> {
>> + struct page *page = folio_page(folio, 0);
>> +
>> + VM_BUG_ON(order > MAX_ORDER_NR_PAGES);
>
> VM_WARN_ON_ONCE() or anything else that is not *BUG, please.
>
Ack
>> +
>> + WARN_ON_ONCE(order && order != HPAGE_PMD_ORDER);
>
> Why do we need that limitation?
>
mTHP is not yet supported in the series. We could keep this routine more generic
and not need the checks, but I added them to prevent unsupported order usage
>> +
>> /*
>> * 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);
>> +
>> + /*
>> + * Only PMD level migration is supported for THP migration
>> + */
>
> I don't understand how that comment interacts with the code below. This is basic large folio initialization.
>
> Drop the comment, or move it above the HPAGE_PMD_ORDER check with a clear reason why that limitation excists.
>
Ack
>> + if (order > 1) {
>> + prep_compound_page(page, order);
>> + folio_set_large_rmappable(folio);
>> + }
>> }
>> -EXPORT_SYMBOL_GPL(zone_device_page_init);
>> +EXPORT_SYMBOL_GPL(init_zone_device_folio);
>
>
Thanks for the review
Balbir
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