Re: [PATCH v10 2/8] mm: factor zone-device page init helpers out of __init_zone_device_page

["Li Zhe" <lizhe.67@bytedance.com>]

On 8/16/26 8:04 PM, Mike Rapoport wrote:
> Hi,
>
>> 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_requires_refcount_reset(),
>> 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>
>>
>> diff --git a/mm/mm_init.c b/mm/mm_init.c
>> index 95808ab5cfdb..a70acb7431a6 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 memmap_init_zone_device() must initialize the page
>> + * refcount to 0. MEMORY_DEVICE_GENERIC pages regain a refcount of 1 in
>> + * the free path, while the remaining ZONE_DEVICE types start from 0 here
>> + * and raise the count again when the allocator or driver hands the page
>> + * out.
>> + */
>> +static inline bool pagemap_requires_refcount_reset(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 true;
>> +	case MEMORY_DEVICE_GENERIC:
>> +		return false;
>> +	}
>> +
>> +	return false;
>> +}
>> +
>> +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_requires_refcount_reset(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);
> It looks like all callers of __init_zone_device_page() are now calling
> zone_device_page_init_slow() that with series evolution became a thin
> wrapper for __zone_device_page_init().
>
> Maybe we can just rename __init_zone_device_page() to
> __zone_device_page_init() and replace the open coded setting of the page
> count with the call to pagemap_requires_refcount_reset() an be done
> here?


Yes, that would make patch 2 simpler.

The reason I kept the refcount policy outside __zone_device_page_init()
is the compound-tail initialization added later in patch 5. Tail pages
still need the same basic ZONE_DEVICE struct page initialization as head
pages, including __init_single_page(), PageReserved, pgmap and
zone_device_data setup. After that, the tail-page path calls
prep_compound_tail() and sets the tail refcount to 0 unconditionally.

Folding pagemap_requires_refcount_reset() into __zone_device_page_init()
would make that helper include the head-page refcount policy even for
callers that will immediately apply the tail-page refcount rule
afterwards.

That preserves the final state, but I was trying to keep the common
ZONE_DEVICE page setup separate from the head-page refcount policy for
this reason.

Would you still prefer the simpler renamed helper here, or should I keep
the common ZONE_DEVICE page setup separate from the refcount policy and
make the naming clearer in v11?

Thanks,
Zhe