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Message-ID: <048134fd-6a3d-4a6c-a2eb-9a9911c3b35f@kernel.org>
Date: Wed, 12 Nov 2025 11:00:01 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Balbir Singh <balbirs@...dia.com>, linux-mm@...ck.org
Cc: linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
Zi Yan <ziy@...dia.com>, 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>, Alistair Popple <apopple@...dia.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: [PATCH] mm/huge_memory.c: introduce split_unmapped_folio_to_order
On 12.11.25 05:46, Balbir Singh wrote:
> Unmapped was added as a parameter to __folio_split() and related
> call sites to support splitting of folios already in the midst
> of a migration. This special case arose for device private folio
> migration since during migration there could be a disconnect between
> source and destination on the folio size.
>
> Introduce split_unmapped_folio_to_order() to handle this special case.
> This in turn removes the special casing introduced by the unmapped
> parameter in __folio_split().
As raised recently, I would hope that we can find a way to make all
these splitting functions look more similar in the long term, ideally
starting with "folio_split" / "folio_try_split".
What about
folio_split_unmapped()
Do we really have to spell out the "to order" part in the function name?
And if it's more a mostly-internal helper, maybe
__folio_split_unmapped()
subject: "mm/huge_memory: introduce ..."
>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> 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>
>
> Suggested-by: Zi Yan <ziy@...dia.com>
> Signed-off-by: Balbir Singh <balbirs@...dia.com>
> ---
> include/linux/huge_mm.h | 5 +-
> mm/huge_memory.c | 135 ++++++++++++++++++++++++++++++++++------
> mm/migrate_device.c | 3 +-
> 3 files changed, 120 insertions(+), 23 deletions(-)
>
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index e2e91aa1a042..9155e683c08a 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -371,7 +371,8 @@ enum split_type {
>
> bool can_split_folio(struct folio *folio, int caller_pins, int *pextra_pins);
> int __split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
> - unsigned int new_order, bool unmapped);
> + unsigned int new_order);
> +int split_unmapped_folio_to_order(struct folio *folio, unsigned int new_order);
> int min_order_for_split(struct folio *folio);
> int split_folio_to_list(struct folio *folio, struct list_head *list);
> bool folio_split_supported(struct folio *folio, unsigned int new_order,
> @@ -382,7 +383,7 @@ int folio_split(struct folio *folio, unsigned int new_order, struct page *page,
> static inline int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
> unsigned int new_order)
> {
> - return __split_huge_page_to_list_to_order(page, list, new_order, false);
> + return __split_huge_page_to_list_to_order(page, list, new_order);
> }
> static inline int split_huge_page_to_order(struct page *page, unsigned int new_order)
> {
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 0184cd915f44..942bd8410c54 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -3747,7 +3747,6 @@ bool folio_split_supported(struct folio *folio, unsigned int new_order,
> * @lock_at: a page within @folio to be left locked to caller
> * @list: after-split folios will be put on it if non NULL
> * @split_type: perform uniform split or not (non-uniform split)
> - * @unmapped: The pages are already unmapped, they are migration entries.
> *
> * It calls __split_unmapped_folio() to perform uniform and non-uniform split.
> * It is in charge of checking whether the split is supported or not and
> @@ -3763,7 +3762,7 @@ bool folio_split_supported(struct folio *folio, unsigned int new_order,
> */
> static int __folio_split(struct folio *folio, unsigned int new_order,
> struct page *split_at, struct page *lock_at,
> - struct list_head *list, enum split_type split_type, bool unmapped)
> + struct list_head *list, enum split_type split_type)
Yeah, nice to see that go.
> {
> struct deferred_split *ds_queue;
> XA_STATE(xas, &folio->mapping->i_pages, folio->index);
> @@ -3809,14 +3808,12 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
> * is taken to serialise against parallel split or collapse
> * operations.
> */
> - if (!unmapped) {
> - anon_vma = folio_get_anon_vma(folio);
> - if (!anon_vma) {
> - ret = -EBUSY;
> - goto out;
> - }
> - anon_vma_lock_write(anon_vma);
> + anon_vma = folio_get_anon_vma(folio);
> + if (!anon_vma) {
> + ret = -EBUSY;
> + goto out;
> }
> + anon_vma_lock_write(anon_vma);
> mapping = NULL;
> } else {
> unsigned int min_order;
> @@ -3882,8 +3879,7 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
> goto out_unlock;
> }
>
> - if (!unmapped)
> - unmap_folio(folio);
> + unmap_folio(folio);
>
Hm, I would have hoped that we could factor out the core logic and reuse
it for the new helper, instead of duplicating code.
Did you look into that?
--
Cheers
David
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