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Message-ID: <c782bdbf-de29-44c5-a8ad-619ce89c212d@kernel.org>
Date: Tue, 18 Nov 2025 21:17:07 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Zi Yan <ziy@...dia.com>, Balbir Singh <balbirs@...dia.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
akpm@...ux-foundation.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>, 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 14.11.25 01:23, Zi Yan wrote:
> On 13 Nov 2025, at 16:56, Balbir Singh wrote:
>
>> On 11/14/25 08:45, Zi Yan wrote:
>>> On 13 Nov 2025, at 16:39, Balbir Singh wrote:
>>>
>>>> On 11/13/25 10:49, Balbir Singh wrote:
>>>>> On 11/12/25 22:34, David Hildenbrand (Red Hat) wrote:
>>>>>> 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.
>>>>>>
>>>>>
>>>>> Ack
>>>>>
>>>>>>>
>>>>>>>
>>>>>>>>>
>>>>>>>>> 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.
>>>>>>
>>>>>
>>>>> I came up with a helper, but that ends up with another boolean do_lru.
>>>>>
>>>>>
>>>>
>>>> Zi, David, any opinions on the approach below?
>>>
>>> Looks good to me. We might want a better name instead of
>>> __folio_split_unmapped(). Or __split_unmapped_folio() should
>>> be renamed, since these two function names are too similar.
>>>
>>> Maybe __folio_split_unmapped() -> __freeze_and_split_unmapped_folio().
>>>
>>> Feel free to come up with a better name. :)
>>>
>>
>> I had __folio_split_unfreeze() to indicate we split the folio and unfreeze at the end, but
>> it does not reflect that we freeze it as well. Looks like we are trending towards folio_split
>> as the prefix (IIUC Dave correctly), I like your name, but if folio_split is going to be
>> required then may be __folio_split_unmapped_unfreeze()?
>>
>
> OK, if __folio prefix is a convention,
Yes, let's start cleaning all this up.
> how about
> __folio_freeze_and_split_unmapped()? __folio_split_unmapped_unfreeze() sounds
> like folio is frozen when the function is called. Of course, a more accurate
> one would be __folio_freeze_split_unfreeze_unmapped(). It can work if
> it is not too long. :)
Let me take a look at v2.
--
Cheers
David
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