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Message-ID: <826df5b1-b61d-4794-a96c-dcf9ac19e269@kernel.org>
Date: Wed, 12 Nov 2025 12:34:22 +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 11:17, Balbir Singh wrote:
> 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.
I think we should start naming them more consistently regarding
folio_split() immediately and cleanup the other ones later.
I don't understand why "_to_order" must be in the name right now. You
can add another variant and start using longer names when really required.
>
>
>>>
>>> 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).
Looking at it in more detail, the code duplication is not desired.
We have to find a way to factor the existing code out and reuse it from
any new function.
--
Cheers
David
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