On 9/18/25 22:27, Chris Mason wrote:
>
> [ apologies for resending this, debugging sendmail ]
>
> Hi Balbir,
>
> On Mon, 8 Sep 2025 10:04:34 +1000 Balbir Singh <balb...@nvidia.com> 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.
>
> [ ... ]
>
>> diff --git a/mm/memremap.c b/mm/memremap.c
>> index 46cb1b0b6f72..66f9186b5500 100644
>> --- a/mm/memremap.c
>> +++ b/mm/memremap.c
>> @@ -453,11 +452,15 @@ void free_zone_device_folio(struct folio *folio)
>>
>> switch (pgmap->type) {
>> case MEMORY_DEVICE_PRIVATE:
>> + percpu_ref_put_many(&folio->pgmap->ref, nr);
>
> Here we're dropping nr refs
>
>> + pgmap->ops->page_free(&folio->page);
>> + 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);
>
> Here we're dropping one ref?
>
>> break;
>>
>> case MEMORY_DEVICE_GENERIC:
>> @@ -480,14 +483,23 @@ 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);
>> +
>> + 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));
>
> Here we always bump by 1 << order
>
> I hesitate to send this one because I don't know the code at all, but the
> AI review prompts keep flagging this apparent refcount mismatch, and it looks
> real to me.
>
> Are the differences in refcount handling inside free_zone_device_folio()
> intentional?
>
Thanks for the review! I've posted a v6, but in general
1. Large folios for supported for device-private pages, hence the nr (from
folio_order)
2. The user of the folios specifies the order (for mTHP), a folio gets get
created
with that order
3. What you are seeing with the percpu_ref_put for two different cases
MEMORY_DEVICE_COHERENT and MEMORY_DEVICE_PRIVATE
The reason I point you to v6, is that the diff is a little bit different there
Balbir
