Re: [PATCH linux-next v2] mm: huge_memory: fix misused mapping_large_folio_support() for anon folios

[David Hildenbrand <david@redhat.com>]

On 06.06.24 11:42, 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 the test cases are only for anonmous folios.
> while mapping_large_folio_support() is only reasonable for page
> cache folios.
> 
> 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.
> 
> Also add a VM_WARN_ON_ONCE() in mapping_large_folio_support()
> for anon mapping, So we can detect the wrong use more easily.
> 
> THP folios maybe exist in the pagecache even the file system doesn't
> support large folio, it is because when CONFIG_TRANSPARENT_HUGEPAGE
> is enabled, khugepaged will try to collapse read-only file-backed pages
> to THP. But the mapping does not actually support multi order
> large folios properly.
> 
> Using /sys/kernel/debug/split_huge_pages to verify this, with this
> patch, large anon THP is successfully split and the warning is ceased.
> 

Smaller nits:

> +	} else if (new_order) {
>   		/* 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)) {
> +		/* No split if the file system does not support large folio.

/*
  * No ...


> +		 * Note that we might still have THPs in such mappings due to
> +		 * CONFIG_READ_ONLY_THP_FOR_FS. But in that case, the mapping
> +		 * does not actually support large folios properly.
> +		 */
> +		if (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) &&
> +			!mapping_large_folio_support(folio->mapping)) {

if (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) &&
     !mapping_large_folio_support(folio->mapping)) {

>   			VM_WARN_ONCE(1,
>   				"Cannot split file folio to non-0 order");
>   			return -EINVAL;
>   		}
>   	}
> 
> +	/* Only swapping a whole PMD-mapped folio is supported */
> +	if (folio_test_swapcache(folio) && new_order)
> +		return -EINVAL;
> 
>   	is_hzp = is_huge_zero_folio(folio);
>   	if (is_hzp) {

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb