linux-kernel - Re: [PATCH] mm/huge_memory.c: introduce folio_split

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Message-ID: <870151ce-ca90-4cd4-8f21-35f4da329924@nvidia.com>
Date: Fri, 14 Nov 2025 14:30:03 +1100
From: Balbir Singh <balbirs@...dia.com>
To: Wei Yang <richard.weiyang@...il.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 dri-devel@...ts.freedesktop.org, Andrew Morton <akpm@...ux-foundation.org>,
 David Hildenbrand <david@...hat.com>, 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 folio_split_unmapped

On 11/14/25 14:21, Wei Yang wrote:
> On Fri, Nov 14, 2025 at 12:22:28PM +1100, Balbir Singh wrote:
> [...]
>> @@ -4079,6 +4091,36 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>> 	return ret;
>> }
>>
>> +/*
>> + * This function is a helper for splitting folios that have already been unmapped.
>> + * The use case is that the device or the CPU can refuse to migrate THP pages in
>> + * the middle of migration, due to allocation issues on either side
>> + *
>> + * The high level code is copied from __folio_split, since the pages are anonymous
>> + * and are already isolated from the LRU, the code has been simplified to not
>> + * burden __folio_split with unmapped sprinkled into the code.
>> + *
>> + * None of the split folios are unlocked
>> + */
>> +int folio_split_unmapped(struct folio *folio, unsigned int new_order)
>> +{
>> +	int extra_pins, ret = 0;
>> +
>> +	VM_WARN_ON_FOLIO(folio_mapped(folio), folio);
>> +	VM_WARN_ON_ONCE_FOLIO(!folio_test_locked(folio), folio);
>> +	VM_WARN_ON_ONCE_FOLIO(!folio_test_large(folio), folio);
>> +
> 
> Compared with original logic, we did check folio_split_supported() and
> check whether new_order is supported for the file system.
> 
> Currently folio_split_unmapped() only pass 0 as new_order, which looks good.
> But for a generic helper, it looks reasonable to do the check, IMHO.
> 

This is meant to be used in the middle of a migration where the src/dst do 
no agree on the folio_order() due to allocation issues. When mTHP support
is added to device migration, order support will be added and checked.
FYI: This routines supports just anonymous pages ATM

>> +	if (!can_split_folio(folio, 1, &extra_pins))
>> +		return -EAGAIN;
>> +
>> +	local_irq_disable();
>> +	ret = __folio_freeze_and_split_unmapped(folio, new_order, &folio->page, NULL,
>> +						NULL, false, NULL, SPLIT_TYPE_UNIFORM,
>> +						0, extra_pins);
>> +	local_irq_enable();
>> +	return ret;
>> +}
>> +
>> /*
>>  * This function splits a large folio into smaller folios of order @new_order.
>>  * @page can point to any page of the large folio to split. The split operation
> 
> 

Balbir