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Message-ID: <882D81FA-DA40-4FF9-8192-166DBE1709AF@nvidia.com>
Date: Wed, 17 Sep 2025 22:49:36 -0400
From: Zi Yan <ziy@...dia.com>
To: Balbir Singh <balbirs@...dia.com>, David Hildenbrand <david@...hat.com>,
Alistair Popple <apopple@...dia.com>
Cc: 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 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.
> }
> -EXPORT_SYMBOL_GPL(zone_device_page_init);
> +EXPORT_SYMBOL_GPL(zone_device_folio_init);
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 34333ae3bd80..9a2aabfaea6f 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1769,9 +1769,13 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
> * the folio is unmapped and at least one page is still mapped.
> *
> * Check partially_mapped first to ensure it is a large folio.
> + *
> + * Device private folios do not support deferred splitting and
> + * shrinker based scanning of the folios to free.
> */
> if (partially_mapped && folio_test_anon(folio) &&
> - !folio_test_partially_mapped(folio))
> + !folio_test_partially_mapped(folio) &&
> + !folio_is_device_private(folio))
> deferred_split_folio(folio, true);
>
> __folio_mod_stat(folio, -nr, -nr_pmdmapped);
> --
> 2.50.1
--
Best Regards,
Yan, Zi
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