Re: [PATCH linux-next] mm: huge_memory: fix misused mapping_large_folio_support() for anon folios

[David Hildenbrand <david@redhat.com>]

On 04.06.24 07:47, xu.xin16@zte.com.cn wrote:
> From: Ran Xiaokai <ran.xiaokai@zte.com.cn>
> 
> When I did a large folios split test, a WARNING
> "[ 5059.122759][  T166] Cannot split file folio to non-0 order"
> was triggered. But my test cases are only for anonmous folios.
> while mapping_large_folio_support() is only reasonable for page
> cache folios.

Agreed.

I wonder if mapping_large_folio_support() should either

a) Complain if used for anon folios, so we can detect the wrong use more 
easily. (VM_WARN_ON_ONCE())

b) Return "true" for anonymous mappings, although that's more debatable.

> 
> In split_huge_page_to_list_to_order(), the folio passed to
> mapping_large_folio_support() maybe anonmous folio. The
> folio_test_anon() check is missing. So the split of the anonmous THP
> is failed. This is also the same for shmem_mapping(). We'd better add
> a check for both. But the shmem_mapping() in __split_huge_page() is
> not involved, as for anonmous folios, the end parameter is set to -1, so
> (head[i].index >= end) is always false. shmem_mapping() is not called.
> 
> Using /sys/kernel/debug/split_huge_pages to verify this, with this
> patch, large anon THP is successfully split and the warning is ceased.
> 
> Signed-off-by: Ran Xiaokai <ran.xiaokai@zte.com.cn>
> Cc: xu xin <xu.xin16@zte.com.cn>
> Cc: Yang Yang <yang.yang29@zte.com.cn>
> ---
>   mm/huge_memory.c | 38 ++++++++++++++++++++------------------
>   1 file changed, 20 insertions(+), 18 deletions(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 317de2afd371..4c9c7e5ea20c 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -3009,31 +3009,33 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
>   	if (new_order >= folio_order(folio))
>   		return -EINVAL;
> 
> -	/* Cannot split anonymous THP to order-1 */
> -	if (new_order == 1 && folio_test_anon(folio)) {
> -		VM_WARN_ONCE(1, "Cannot split to order-1 folio");
> -		return -EINVAL;
> -	}
> -
>   	if (new_order) {
>   		/* Only swapping a whole PMD-mapped folio is supported */
>   		if (folio_test_swapcache(folio))
>   			return -EINVAL;
> -		/* Split shmem folio to non-zero order not supported */
> -		if (shmem_mapping(folio->mapping)) {
> -			VM_WARN_ONCE(1,
> -				"Cannot split shmem folio to non-0 order");
> -			return -EINVAL;
> -		}
> -		/* No split if the file system does not support large folio */
> -		if (!mapping_large_folio_support(folio->mapping)) {
> -			VM_WARN_ONCE(1,
> -				"Cannot split file folio to non-0 order");
> -			return -EINVAL;
> +
> +		if (folio_test_anon(folio)) {
> +			/* Cannot split anonymous THP to order-1 */
> +			if (new_order == 1) {
> +				VM_WARN_ONCE(1, "Cannot split to order-1 folio");
> +				return -EINVAL;
> +			}
> +		} else {
> +			/* Split shmem folio to non-zero order not supported */
> +			if (shmem_mapping(folio->mapping)) {
> +				VM_WARN_ONCE(1,
> +					"Cannot split shmem folio to non-0 order");
> +				return -EINVAL;
> +			}
> +			/* No split if the file system does not support large folio */
> +			if (!mapping_large_folio_support(folio->mapping)) {
> +				VM_WARN_ONCE(1,
> +					"Cannot split file folio to non-0 order");
> +				return -EINVAL;
> +			}
>   		}
>   	}

What about the following sequence:

if (folio_test_anon(folio)) {
	if (new_order == 1)
		...
} else if (new_order) {
	if (shmem_mapping(...))
		...
	...
}

if (folio_test_swapcache(folio) && new_order)
	return -EINVAL;

Should result in less churn and reduce indentation level.

-- 
Cheers,

David / dhildenb