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Message-ID: <1b6741fe-20ed-4a47-8e9b-24bd199f23f4@redhat.com>
Date: Thu, 25 Sep 2025 11:43:48 +0200
From: David Hildenbrand <david@...hat.com>
To: Alistair Popple <apopple@...dia.com>, Zi Yan <ziy@...dia.com>
Cc: Balbir Singh <balbirs@...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 25.09.25 01:58, Alistair Popple wrote:
> On 2025-09-25 at 03:36 +1000, Zi Yan <ziy@...dia.com> wrote...
>> On 24 Sep 2025, at 6:55, David Hildenbrand wrote:
>>
>>> On 18.09.25 04:49, 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);
>>>> }
>>>
>>> I think the problem is that it will all be weird once we dynamically allocate "struct folio".
>>>
>>> I have not yet a clear understanding on how that would really work.
>>>
>>> For example, should it be pgmap->ops->page_folio() ?
>>>
>>> Who allocates the folio? Do we allocate all order-0 folios initially, to then merge them when constructing large folios? How do we manage the "struct folio" during such merging splitting?
>>
>> Right. Either we would waste memory by simply concatenating all “struct folio”
>> and putting paddings at the end, or we would free tail “struct folio” first,
>> then allocate tail “struct page”. Both are painful and do not match core mm’s
>> memdesc pattern, where “struct folio” is allocated when caller is asking
>> for a folio. If “struct folio” is always allocated, there is no difference
>> between “struct folio” and “struct page”.
>
> As mentioned in my other reply I need to investigate this some more, but I
> don't think we _need_ to always allocate folios (or pages for that matter).
> The ZONE_DEVICE code just uses folios/pages for interacting with the core mm,
> not for managing the device memory itself, so we should be able to make it more
> closely match the memdesc pattern. It's just I'm still a bit unsure what that
> pattern will actually look like.
I think one reason might be that in contrast to ordinary pages,
zone-device memory is only ever used to be used for folios, right?
Would there be a user that just allocates pages and not wants a folio
associated with it?
It's a good question of that would look like when we have dynamically
allocated struct folio ...
>
>>>
>>> With that in mind, I don't really know what the proper interface should be today.
>>>
>>>
>>> zone_device_folio_init(struct page *page, unsigned int order)
>>>
>>> looks cleaner, agreed.
>
> Agreed.
>
>>>>
>>>>
>>>> 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().
>
> Where would the order parameter come from? Presumably
> folio_order(compound_head(page)) in which case shouldn't the op actually just be
> pgmap->ops->folio_free()?
Yeah, that's also what I thought.
--
Cheers
David / dhildenb
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