Re: [PATCH v4] mm: Use a folio in the softleaf_is_device_private path

[David Hildenbrand (Arm)]

On 8/18/26 04:13, Hongfu Li wrote:
>>> So after migrate_to_ram() the folio might have been split or otherwise somehow
>>> the page doesn't belong to the same locked, refcount-incremented folio it did
>>> before?
>>
>> I suspect a split.
>>
>>>
>>> That's kinda a footgun... but this documents it at least.
>>>
>>> I think a comment explaining how this can happen would be helpful though as this
>>> doesn't seem intuitive.
>>
>> I think, conceptually, calling into something that consumes a page (vmf->page)
>> always needs care when operating on folios.
>>
>> Passing the vmf to some callback might be the odd thing here, because the
>> vmf->page contract is not really clear.
> 
> I'm very sorry for introducing this regression.
> Thank you for identifying and testing this issue.
> 
> I will double-check this code path and apply your fix to run further
> tests locally.
> 
> Would it make sense to add a comment here like the below, to make this
> subtle behavior clearer for future readers?
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 4134ac607ee0..2b6d6c863ecb 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4937,6 +4937,12 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>                                 pte_unmap_unlock(vmf->pte, vmf->ptl);
>                                 pgmap = page_pgmap(vmf->page);
>                                 ret = pgmap->ops->migrate_to_ram(vmf);
> +                               /*
> +                                * migrate_to_ram() can split a large folio, updating
> +                                * vmf->page to a different folio. Re-fetch folio for
> +                                * correct unlock/put.
> +                                */

"migrate_to_ram() might have split the folio."

Should be sufficient I guess.

-- 
Cheers,

David