Message-ID: <4534DB6E-FF66-4412-B843-FB9BC5E52618@nvidia.com>
Date: Wed, 24 Sep 2025 13:49:41 -0400
From: Zi Yan <ziy@...dia.com>
To: David Hildenbrand <david@...hat.com>
Cc: Balbir Singh <balbirs@...dia.com>, Alistair Popple <apopple@...dia.com>,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org, damon@...ts.linux.dev,
 dri-devel@...ts.freedesktop.org, Joshua Hahn <joshua.hahnjy@...il.com>,
 Rakie Kim <rakie.kim@...com>, Byungchul Park <byungchul@...com>,
 Gregory Price <gourry@...rry.net>, Ying Huang <ying.huang@...ux.alibaba.com>,
 Oscar Salvador <osalvador@...e.de>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>, Nico Pache <npache@...hat.com>,
 Ryan Roberts <ryan.roberts@....com>, Dev Jain <dev.jain@....com>,
 Barry Song <baohua@...nel.org>, Lyude Paul <lyude@...hat.com>,
 Danilo Krummrich <dakr@...nel.org>, David Airlie <airlied@...il.com>,
 Simona Vetter <simona@...ll.ch>, Ralph Campbell <rcampbell@...dia.com>,
 Mika Penttilä <mpenttil@...hat.com>,
 Matthew Brost <matthew.brost@...el.com>,
 Francois Dugast <francois.dugast@...el.com>
Subject: Re: [v6 01/15] mm/zone_device: support large zone device private
 folios

On 24 Sep 2025, at 7:04, David Hildenbrand wrote:

> On 23.09.25 05:47, Balbir Singh wrote:
>> On 9/19/25 23:26, Zi Yan wrote:
>>> On 19 Sep 2025, at 1:01, Balbir Singh wrote:
>>>
>>>> On 9/18/25 12:49, Zi Yan wrote:
>>>>> On 16 Sep 2025, at 8:21, Balbir Singh 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.
>>>>>>
>>>>>> Signed-off-by: Balbir Singh <balbirs@...dia.com>
>>>>>> Cc: David Hildenbrand <david@...hat.com>
>>>>>> Cc: Zi Yan <ziy@...dia.com>
>>>>>> Cc: Joshua Hahn <joshua.hahnjy@...il.com>
>>>>>> Cc: Rakie Kim <rakie.kim@...com>
>>>>>> Cc: Byungchul Park <byungchul@...com>
>>>>>> Cc: Gregory Price <gourry@...rry.net>
>>>>>> Cc: Ying Huang <ying.huang@...ux.alibaba.com>
>>>>>> Cc: Alistair Popple <apopple@...dia.com>
>>>>>> Cc: Oscar Salvador <osalvador@...e.de>
>>>>>> Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
>>>>>> Cc: Baolin Wang <baolin.wang@...ux.alibaba.com>
>>>>>> Cc: "Liam R. Howlett" <Liam.Howlett@...cle.com>
>>>>>> Cc: Nico Pache <npache@...hat.com>
>>>>>> Cc: Ryan Roberts <ryan.roberts@....com>
>>>>>> Cc: Dev Jain <dev.jain@....com>
>>>>>> Cc: Barry Song <baohua@...nel.org>
>>>>>> Cc: Lyude Paul <lyude@...hat.com>
>>>>>> Cc: Danilo Krummrich <dakr@...nel.org>
>>>>>> Cc: David Airlie <airlied@...il.com>
>>>>>> Cc: Simona Vetter <simona@...ll.ch>
>>>>>> Cc: Ralph Campbell <rcampbell@...dia.com>
>>>>>> Cc: Mika Penttilä <mpenttil@...hat.com>
>>>>>> Cc: Matthew Brost <matthew.brost@...el.com>
>>>>>> Cc: Francois Dugast <francois.dugast@...el.com>
>>>>>> ---
>>>>>>   include/linux/memremap.h | 10 +++++++++-
>>>>>>   mm/memremap.c            | 34 +++++++++++++++++++++-------------
>>>>>>   mm/rmap.c                |  6 +++++-
>>>>>>   3 files changed, 35 insertions(+), 15 deletions(-)
>>>>>>
>>>>>> diff --git a/include/linux/memremap.h b/include/linux/memremap.h
>>>>>> index e5951ba12a28..9c20327c2be5 100644
>>>>>> --- a/include/linux/memremap.h
>>>>>> +++ b/include/linux/memremap.h
>>>>>> @@ -206,7 +206,7 @@ static inline bool is_fsdax_page(const struct page *page)
>>>>>>   }
>>>>>>
>>>>>>   #ifdef CONFIG_ZONE_DEVICE
>>>>>> -void zone_device_page_init(struct page *page);
>>>>>> +void zone_device_folio_init(struct folio *folio, unsigned int order);
>>>>>>   void *memremap_pages(struct dev_pagemap *pgmap, int nid);
>>>>>>   void memunmap_pages(struct dev_pagemap *pgmap);
>>>>>>   void *devm_memremap_pages(struct device *dev, struct dev_pagemap *pgmap);
>>>>>> @@ -215,6 +215,14 @@ struct dev_pagemap *get_dev_pagemap(unsigned long pfn);
>>>>>>   bool pgmap_pfn_valid(struct dev_pagemap *pgmap, unsigned long pfn);
>>>>>>
>>>>>>   unsigned long memremap_compat_align(void);
>>>>>> +
>>>>>> +static inline void zone_device_page_init(struct page *page)
>>>>>> +{
>>>>>> +	struct folio *folio = page_folio(page);
>>>>>> +
>>>>>> +	zone_device_folio_init(folio, 0);
>>>>>
>>>>> I assume it is for legacy code, where only non-compound page exists?
>>>>>
>>>>> It seems that you assume @page is always order-0, but there is no check
>>>>> for it. Adding VM_WARN_ON_ONCE_FOLIO(folio_order(folio) != 0, folio)
>>>>> above it would be useful to detect misuse.
>>>>>
>>>>>> +}
>>>>>> +
>>>>>>   #else
>>>>>>   static inline void *devm_memremap_pages(struct device *dev,
>>>>>>   		struct dev_pagemap *pgmap)
>>>>>> diff --git a/mm/memremap.c b/mm/memremap.c
>>>>>> index 46cb1b0b6f72..a8481ebf94cc 100644
>>>>>> --- a/mm/memremap.c
>>>>>> +++ b/mm/memremap.c
>>>>>> @@ -416,20 +416,19 @@ EXPORT_SYMBOL_GPL(get_dev_pagemap);
>>>>>>   void free_zone_device_folio(struct folio *folio)
>>>>>>   {
>>>>>>   	struct dev_pagemap *pgmap = folio->pgmap;
>>>>>> +	unsigned long nr = folio_nr_pages(folio);
>>>>>> +	int i;
>>>>>>
>>>>>>   	if (WARN_ON_ONCE(!pgmap))
>>>>>>   		return;
>>>>>>
>>>>>>   	mem_cgroup_uncharge(folio);
>>>>>>
>>>>>> -	/*
>>>>>> -	 * Note: we don't expect anonymous compound pages yet. Once supported
>>>>>> -	 * and we could PTE-map them similar to THP, we'd have to clear
>>>>>> -	 * PG_anon_exclusive on all tail pages.
>>>>>> -	 */
>>>>>>   	if (folio_test_anon(folio)) {
>>>>>> -		VM_BUG_ON_FOLIO(folio_test_large(folio), folio);
>>>>>> -		__ClearPageAnonExclusive(folio_page(folio, 0));
>>>>>> +		for (i = 0; i < nr; i++)
>>>>>> +			__ClearPageAnonExclusive(folio_page(folio, i));
>>>>>> +	} else {
>>>>>> +		VM_WARN_ON_ONCE(folio_test_large(folio));
>>>>>>   	}
>>>>>>
>>>>>>   	/*
>>>>>> @@ -456,8 +455,8 @@ void free_zone_device_folio(struct folio *folio)
>>>>>>   	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_many(&folio->pgmap->ref, nr);
>>>>>>   		break;
>>>>>>
>>>>>>   	case MEMORY_DEVICE_GENERIC:
>>>>>> @@ -480,14 +479,23 @@ 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);
>>>>>
>>>>> It is strange to see a folio is converted back to page in
>>>>> a function called zone_device_folio_init().
>>>>>
>>>>>> +
>>>>>> +	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));
>>>>>> +	folio_set_count(folio, 1);
>>>>>>   	lock_page(page);
>>>>>> +
>>>>>> +	if (order > 1) {
>>>>>> +		prep_compound_page(page, order);
>>>>>> +		folio_set_large_rmappable(folio);
>>>>>> +	}
>>>>>
>>>>> OK, so basically, @folio is not a compound page yet when zone_device_folio_init()
>>>>> is called.
>>>>>
>>>>> I feel that your zone_device_page_init() and zone_device_folio_init()
>>>>> implementations are inverse. They should follow the same pattern
>>>>> as __alloc_pages_noprof() and __folio_alloc_noprof(), where
>>>>> zone_device_page_init() does the actual initialization and
>>>>> zone_device_folio_init() just convert a page to folio.
>>>>>
>>>>> Something like:
>>>>>
>>>>> void zone_device_page_init(struct page *page, unsigned int order)
>>>>> {
>>>>> 	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_many(&page_pgmap(page)->ref, 1 << order));
>>>>> 	
>>>>> 	/*
>>>>> 	 * anonymous folio does not support order-1, high order file-backed folio
>>>>> 	 * is not supported at all.
>>>>> 	 */
>>>>> 	VM_WARN_ON_ONCE(order == 1);
>>>>>
>>>>> 	if (order > 1)
>>>>> 		prep_compound_page(page, order);
>>>>>
>>>>> 	/* page has to be compound head here */
>>>>> 	set_page_count(page, 1);
>>>>> 	lock_page(page);
>>>>> }
>>>>>
>>>>> void zone_device_folio_init(struct folio *folio, unsigned int order)
>>>>> {
>>>>> 	struct page *page = folio_page(folio, 0);
>>>>>
>>>>> 	zone_device_page_init(page, order);
>>>>> 	page_rmappable_folio(page);
>>>>> }
>>>>>
>>>>> Or
>>>>>
>>>>> struct folio *zone_device_folio_init(struct page *page, unsigned int order)
>>>>> {
>>>>> 	zone_device_page_init(page, order);
>>>>> 	return page_rmappable_folio(page);
>>>>> }
>>>>>
>>>>>
>>>>> Then, it comes to free_zone_device_folio() above,
>>>>> I feel that pgmap->ops->page_free() should take an additional order
>>>>> parameter to free a compound page like free_frozen_pages().
>>>>>
>>>>>
>>>>> This is my impression after reading the patch and zone device page code.
>>>>>
>>>>> Alistair and David can correct me if this is wrong, since I am new to
>>>>> zone device page code.
>>>>> 	
>>>>
>>>> Thanks, I did not want to change zone_device_page_init() for several
>>>> drivers (outside my test scope) that already assume it has an order size of 0.
>>>
>>> But my proposed zone_device_page_init() should still work for order-0
>>> pages. You just need to change call site to add 0 as a new parameter.
>>>
>>
>> I did not want to change existing callers (increases testing impact)
>> without a strong reason.
>>
>>>
>>> One strange thing I found in the original zone_device_page_init() is
>>> the use of page_pgmap() in
>>> WARN_ON_ONCE(!percpu_ref_tryget_many(&page_pgmap(page)->ref, 1 << order)).
>>> page_pgmap() calls page_folio() on the given page to access pgmap field.
>>> And pgmap field is only available in struct folio. The code initializes
>>> struct page, but in middle it suddenly finds the page is actually a folio,
>>> then treat it as a page afterwards. I wonder if it can be done better.
>>>
>>> This might be a question to Alistair, since he made the change.
>>>
>>
>> I'll let him answer it :)
>
> Not him, but I think this goes back to my question raised in my other reply: When would we allocate "struct folio" in the future.
>
> If it's "always" then actually most of the zone-device code would only ever operate on folios and never on pages in the future.
>
> I recall during a discussion at LSF/MM I raised that, and the answer was (IIRC) that we will allocate "struct folio" as we will initialize the memmap for dax.
>
> So essentially, we'd always have folios and would never really have to operate on pages.

Hmm, then what is the point of having “struct folio”, which originally is
added to save compound_head() calls, where everything is a folio in device
private world? We might need DAX people to explain the rationale of
“always struct folio”.

Best Regards,
Yan, Zi

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