Re: [PATCH v6 1/5] mm/zone_device: Reinitialize large zone device private folios

[Zi Yan <ziy@nvidia.com>]

On 19 Jan 2026, at 17:15, Balbir Singh wrote:

> On 1/20/26 07:35, Jason Gunthorpe wrote:
>> On Mon, Jan 19, 2026 at 03:09:00PM -0500, Zi Yan wrote:
>>>> diff --git a/mm/internal.h b/mm/internal.h
>>>> index e430da900430a1..a7d3f5e4b85e49 100644
>>>> --- a/mm/internal.h
>>>> +++ b/mm/internal.h
>>>> @@ -806,14 +806,21 @@ static inline void prep_compound_head(struct page *page, unsigned int order)
>>>>  		atomic_set(&folio->_pincount, 0);
>>>>  		atomic_set(&folio->_entire_mapcount, -1);
>>>>  	}
>>>> -	if (order > 1)
>>>> +	if (order > 1) {
>>>>  		INIT_LIST_HEAD(&folio->_deferred_list);
>>>> +	} else {
>>>> +		folio->mapping = NULL;
>>>> +#ifdef CONFIG_MEMCG
>>>> +		folio->memcg_data = 0;
>>>> +#endif
>>>> +	}
>>>
>>> prep_compound_head() is only called on >0 order pages. The above
>>> code means when order == 1, folio->mapping and folio->memcg_data are
>>> assigned NULL.
>>
>> OK, fair enough, the conditionals would have to change and maybe it
>> shouldn't be called "compound_head" if it also cleans up normal pages.
>>
>>>>  static inline void prep_compound_tail(struct page *head, int tail_idx)
>>>>  {
>>>>  	struct page *p = head + tail_idx;
>>>>
>>>> +	p->flags.f &= ~0xffUL;	/* Clear possible order, page head */
>>>
>>> No one cares about tail page flags if it is not checked in check_new_page()
>>> from mm/page_alloc.c.
>>
>> At least page_fixed_fake_head() does check PG_head in some
>> configurations. It does seem safer to clear it. Possibly order is
>> never used, but it is free to clear it.
>>
>>>> -	if (order)
>>>> -		prep_compound_page(page, order);
>>>> +	prep_compound_page(page, order);
>>>
>>> prep_compound_page() should only be called for >0 order pages. This creates
>>> another weirdness in device pages by assuming all pages are
>>> compound.
>>
>> OK
>>
>>>> +	folio = page_folio(page);
>>>> +	folio->pgmap = pgmap;
>>>> +	folio_lock(folio);
>>>> +	folio_set_count(folio, 1);
>>>
>>> /* clear possible previous page->mapping */
>>> folio->mapping = NULL;
>>>
>>> /* clear possible previous page->_nr_pages */
>>> #ifdef CONFIG_MEMCG
>>> 	folio->memcg_data = 0;
>>> #endif
>>
>> This is reasonable too, but prep_compound_head() was doing more than
>> that, it is also clearing the order, and this needs to clear the head
>> bit.  That's why it was apppealing to reuse those functions, but you
>> are right they are not ideal.

PG_head is and must be bit 6, that means the stored order needs to be
at least 2^6=64 to get it set. Who allocates a folio with that large order?
This p->flags.f &= ~0xffUL thing is unnecessary. What really needs
to be done is folio->flags.f &= ~PAGE_FLAGS_CHECK_AT_PREP to make
sure the new folio flags are the same as newly allocated folios
from core MM page allocator.

>>
>> I suppose we want some prep_single_page(page) and some reorg to share
>> code with the other prep function.

This is just an unnecessary need due to lack of knowledge of/do not want
to investigate core MM page and folio initialization code.

>>
>
> There is __init_zone_device_page() and __init_single_page(),
> it does zero out the page and sets the zone, pfn, nid among other things.
> I propose we use the current version with zone_device_free_folio() as is.
>
> We can figure out if __init_zone_device_page() can be reused or refactored
> for the purposes to doing this with core MM API's
>
>
>>> This patch mixed the concept of page and folio together, thus
>>> causing confusion. Core MM sees page and folio two separate things:
>>> 1. page is the smallest internal physical memory management unit,
>>> 2. folio is an abstraction on top of pages, and other abstractions can be
>>>    slab, ptdesc, and more (https://kernelnewbies.org/MatthewWilcox/Memdescs).
>>
>> I think the users of zone_device_page_init() are principally trying to
>> create something that can be installed in a non-special PTE. Meaning
>> the output is always a folio because it is going to be read as a folio
>> in the page walkers.
>>
>> Thus, the job of this function is to take the memory range starting at
>> page for 2^order and turn it into a single valid folio with refcount
>> of 1.
>>
>>> If device pages have to initialize on top of pages with obsolete states,
>>> at least it should be first initialized as pages, then as folios to avoid
>>> confusion.
>>
>> I don't think so. It should do the above job efficiently and iterate
>> over the page list exactly once.

folio initialization should not iterate over any page list, since folio is
supposed to be treated as a whole instead of individual pages.

Based on my understanding,

folio->mapping = NULL;
folio->memcg_data = 0;
folio->flags.f &= ~PAGE_FLAGS_CHECK_AT_PREP;

should be enough.

if (order)
	folio_set_large_rmappable(folio);

is done at zone_device_folio_init().

Best Regards,
Yan, Zi