Re: [PATCH] mm/huge_memory.c: introduce folio_split_unmapped

Balbir Singh
2025-11-14T03:30:21Z

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