Re: [v5 01/15] mm/zone_device: support large zone device private folios

[Chris Mason <clm@meta.com>]

[ apologies for resending this, debugging sendmail ]

Hi Balbir,

On Mon,  8 Sep 2025 10:04:34 +1000 Balbir Singh <balbirs@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?

-chris