Re: Possible regression in pin_user_pages_fast() behavior after commit 7ac67301e82f ("ext4: enable large folio for regular file")

[Zhang Yi <yi.zhang@huaweicloud.com>]

On 10/23/2025 3:24 PM, David Hildenbrand wrote:
>>> __split_huge_pmd_locked() contains that handling.
>>>
>>> We have to do that because we did not preallocate a page table we can just throw in.
>>>
>>> We could do that on this path instead: remap the PMD to be mapped by a PTE table. We'd have to preallocate a page table.
>>>
>>> That would avoid the do_pte_missing() below for such faults.
>>>
>>> that could be done later on top of this fix.
>>
>> Yeah, thank you for the explanation! I have another question, just curious.
>> Why do we have to fall back to installing the PTE table instead of creating
>> a new anonymous large folio (2M) and setting a new leaf huge PMD?
> 
> Primarily because it would waste more memory for various use cases, on a factor of 512.
> 

Ha, I got it, that makes sense! :-)

Thanks,
Yi.