Re: [PATCH] mm/migrate_device: fix folio refcount leak on folio_split_unmapped failure

[Usama Arif <usama.arif@linux.dev>]

> 
> I do not think there is a need to explain why there is no folio_put()
> below. How about below?
> 
> 1. it makes sure the folio has the right ref count,
> 2. it explains folio_get() is for split_huge_pmd_address() instead of
>    folio_split_unmapped().
> 
> diff --git a/mm/migrate_device.c b/mm/migrate_device.c
> index 0a8b31939640f..0b31b878210ba 100644
> --- a/mm/migrate_device.c
> +++ b/mm/migrate_device.c
> @@ -914,8 +914,14 @@ static int migrate_vma_split_unmapped_folio(struct migrate_vma *migrate,
>  	unsigned long flags;
>  	int ret = 0;
> 
> +	VM_WARN_ON_ONCE(folio_ref_count(folio) == 1);

Thanks! I have sent it in [1]. One thing is, I think you mean over here
folio_ref_count(folio) != 1, right? Thats what the patch has.


[1] https://lore.kernel.org/all/20260306104409.3915811-1-usama.arif@linux.dev/

> +	/*
> +	 * take a reference, since split_huge_pmd_address() with freeze = true
> +	 * drops a reference at the end.
> +	 */
>  	folio_get(folio);
>  	split_huge_pmd_address(migrate->vma, addr, true);
> +	
>  	ret = folio_split_unmapped(folio, 0);
>  	if (ret)
>  		return ret;
> 
> 
>>
>>>>
>>>>
>>>> [1] https://gist.github.com/uarif1/65e1e816af7aa0ae38dd6ec64d62a993
>>>> [2] https://gist.github.com/uarif1/79ea9500667daa4e2ef09cb5d308f041
>>>> [3] https://gist.github.com/uarif1/8a35a6c65ba8b3a1c1dfe72dc30e821d
>>>
>>>
>>> Best Regards,
>>> Yan, Zi
> 
> 
> Best Regards,
> Yan, Zi