Re: [PATCH v6 2/8] mm: factor zone-device page init helpers out of __init_zone_device_page

On 7/14/26 7:35 PM, Balbir Singh wrote:
> On 7/9/26 9:25 PM, 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 <[email protected]>
>> Reviewed-by: Mike Rapoport (Microsoft) <[email protected]>
>> ---
>>   mm/mm_init.c | 56 ++++++++++++++++++++++++++++++++++------------------
>>   1 file changed, 37 insertions(+), 19 deletions(-)
>>
>> diff --git a/mm/mm_init.c b/mm/mm_init.c
>> index 95808ab5cfdb..5fccfbacf855 100644
>> --- a/mm/mm_init.c
>> +++ b/mm/mm_init.c
>> @@ -1005,11 +1005,37 @@ 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)
>> +{
> resets_refcount reads like it resets it to 0, but it could be just me. How does
> pagemap_requires_refcount_reset() sound, with the value returned being toggled


Agreed. pagemap_requires_refcount_reset() is clearer here, especially at
the call site as well. I will rename the helper and flip the return
value in the next version.


>> +	/*
>> +	 * 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;
>> +	}
>> +
>> +	return true;
> Shouldn't the default be a VM_WARN_ON()?

I would prefer not to add a default case here. As David pointed out
earlier in the discussion, keeping the switch exhaustive without a
default preserves compiler checking when new enum memory_type values are
added.

Thanks,
Zhe

>> +}
>> +
>> +static void __ref __zone_device_page_init(struct page *page, unsigned long pfn,
>>   					  unsigned long zone_idx, int nid,
>>   					  struct dev_pagemap *pgmap)
>>   {
>> -
>>   	__init_single_page(page, pfn, zone_idx, nid);
>>   
>>   	/*
>> @@ -1028,23 +1054,15 @@ static void __ref __init_zone_device_page(struct page *page, unsigned long pfn,
>>   	 */
>>   	page_folio(page)->pgmap = pgmap;
>>   	page->zone_device_data = NULL;
>> +}
>>   
>> -	/*
>> -	 * 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:
>> +static void __ref zone_device_page_init_slow(struct page *page,
>> +		unsigned long pfn, unsigned long zone_idx, int nid,
>> +		struct dev_pagemap *pgmap)
>> +{
>> +	__zone_device_page_init(page, pfn, zone_idx, nid, pgmap);
>> +	if (!pagemap_resets_refcount(pgmap))
>>   		set_page_count(page, 0);
>> -		break;
>> -
>> -	case MEMORY_DEVICE_GENERIC:
>> -		break;
>> -	}
>>   }
>>   
>>   /*
>> @@ -1090,7 +1108,7 @@ static void __ref memmap_init_compound(struct page *head,
>>   	for (pfn = head_pfn + 1; pfn < end_pfn; pfn++) {
>>   		struct page *page = pfn_to_page(pfn);
>>   
>> -		__init_zone_device_page(page, pfn, zone_idx, nid, pgmap);
>> +		zone_device_page_init_slow(page, pfn, zone_idx, nid, pgmap);
>>   		prep_compound_tail(page, head, order);
>>   		set_page_count(page, 0);
>>   	}
>> @@ -1126,7 +1144,7 @@ void __ref memmap_init_zone_device(struct zone *zone,
>>   	for (pfn = start_pfn; pfn < end_pfn; pfn += pfns_per_compound) {
>>   		struct page *page = pfn_to_page(pfn);
>>   
>> -		__init_zone_device_page(page, pfn, zone_idx, nid, pgmap);
>> +		zone_device_page_init_slow(page, pfn, zone_idx, nid, pgmap);
>>   
>>   		if (IS_ALIGNED(pfn, PAGES_PER_SECTION))
>>   			cond_resched();
> Balbir