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Message-ID: <efa37c4d-8c9f-4094-a713-4fd4452ae917@nvidia.com>
Date: Wed, 12 Nov 2025 21:17:54 +1100
From: Balbir Singh <balbirs@...dia.com>
To: "David Hildenbrand (Red Hat)" <david@...nel.org>, 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 11/12/25 21:00, David Hildenbrand (Red Hat) wrote:
> 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 ..."
>
I can rename it, but currently it confirms to the split_folio with order in the name
The order is there in the name because in the future with mTHP we will want to
support splitting to various orders.
>>
>> 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?
>
>
I did, I ended up with larger spaghetti, I was hoping to look it as a follow up
after the series with the mTHP changes and support (that is to be designed and
prototyped).
Balbir
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