Re: [PATCH v5 2/8] mm: factor zone-device page init helpers out of __init_zone_device_page

["David Hildenbrand (Arm)" <david@kernel.org>]

On 7/1/26 11:05, Li Zhe wrote:
> memmap_init_zone_device() currently mixes refcount policy and core
> ZONE_DEVICE page setup in a single helper.
> 
> Factor the refcount-reset predicate into pagemap_resets_refcount(), move
> the common page initialization into __zone_device_page_init(), and wrap
> the existing slow path in zone_device_page_init_slow().
> 
> This keeps the slow-path behaviour unchanged and gives later patches
> reusable helper boundaries.
> 
> No functional change intended.
> 
> Signed-off-by: Li Zhe <lizhe.67@bytedance.com>
> Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
> ---
>  mm/mm_init.c | 57 ++++++++++++++++++++++++++++++++++------------------
>  1 file changed, 38 insertions(+), 19 deletions(-)
> 
> diff --git a/mm/mm_init.c b/mm/mm_init.c
> index 95808ab5cfdb..4c7fad440c2a 100644
> --- a/mm/mm_init.c
> +++ b/mm/mm_init.c
> @@ -1005,11 +1005,38 @@ static void __init memmap_init(void)
>  }
>  
>  #ifdef CONFIG_ZONE_DEVICE
> -static void __ref __init_zone_device_page(struct page *page, unsigned long pfn,
> +/*
> + * Return true when the free path for this pagemap type restores the page
> + * refcount to 1, so memmap_init_zone_device() can keep the count set by
> + * __init_single_page(). Otherwise initialize the refcount to 0 and leave
> + * it to the allocator or pgmap callbacks to raise it when the page is
> + * handed out again.
> + */
> +static inline bool pagemap_resets_refcount(const struct dev_pagemap *pgmap)
> +{
> +	/*
> +	 * MEMORY_DEVICE_GENERIC pages regain a refcount of 1 in the free
> +	 * path. The remaining ZONE_DEVICE types start from 0 here and raise
> +	 * the count again when the allocator or driver hands the page out.
> +	 */
> +	switch (pgmap->type) {
> +	case MEMORY_DEVICE_FS_DAX:
> +	case MEMORY_DEVICE_PRIVATE:
> +	case MEMORY_DEVICE_COHERENT:
> +	case MEMORY_DEVICE_PCI_P2PDMA:
> +		return false;
> +	case MEMORY_DEVICE_GENERIC:
> +		return true;
> +	default:
> +		WARN_ONCE(1, "Unknown memory type!");
> +		return true;

Wouldn't the compiler warn if we would define a new type but forgot to update it
here? We're using an enum, and I thought the compiler would bail out in that case.

Or are we scared of some other garbage ending up in there?

Apart from that LGTM.

-- 
Cheers,

David