Re: [PATCH 7.2 v4 01/12] mm/khugepaged: remove READ_ONLY_THP_FOR_FS check

["David Hildenbrand (Arm)" <david@kernel.org>]

On 4/24/26 04:49, Zi Yan wrote:
> collapse_file() requires FSes supporting large folio with at least
> PMD_ORDER, so replace the READ_ONLY_THP_FOR_FS check with that.
> MADV_COLLAPSE ignores shmem huge config, so exclude the check for shmem.
> 
> While at it, replace VM_BUG_ON with VM_WARN_ON_ONCE.
> 
> Add a helper function mapping_pmd_thp_support() for FSes supporting large
> folio with at least PMD_ORDER.
> 
> Signed-off-by: Zi Yan <ziy@nvidia.com>
> Reviewed-by: Lance Yang <lance.yang@linux.dev>
> Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> ---
>  include/linux/pagemap.h |  9 +++++++++
>  mm/khugepaged.c         | 10 ++++++++--
>  2 files changed, 17 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index 31a848485ad9..5b4313d91137 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -513,6 +513,15 @@ static inline bool mapping_large_folio_support(const struct address_space *mappi
>  	return mapping_max_folio_order(mapping) > 0;
>  }
>  
> +static inline bool mapping_pmd_thp_support(const struct address_space *mapping)
> +{
> +	/* AS_FOLIO_ORDER is only reasonable for pagecache folios */
> +	VM_WARN_ON_ONCE((unsigned long)mapping & FOLIO_MAPPING_ANON);
> +
> +	return mapping_max_folio_order(mapping) >= PMD_ORDER;
> +}

Given mapping_large_folio_support(), I wonder whether we should call that
mapping_pmd_folio_support() ?

> +
> +
>  /* Return the maximum folio size for this pagecache mapping, in bytes. */
>  static inline size_t mapping_max_folio_size(const struct address_space *mapping)
>  {
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 7d48d4fbd5f3..79f051eb6195 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -2235,8 +2235,14 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,
>  	int nr_none = 0;
>  	bool is_shmem = shmem_file(file);
>  
> -	VM_BUG_ON(!IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) && !is_shmem);
> -	VM_BUG_ON(start & (HPAGE_PMD_NR - 1));
> +	/*
> +	 * MADV_COLLAPSE ignores shmem huge config, so do not check shmem
> +	 *
> +	 * TODO: once shmem always calls mapping_set_large_folios() on its
> +	 * mapping, the shmem check can be removed.
> +	 */
> +	VM_WARN_ON_ONCE(!is_shmem && !mapping_pmd_thp_support(mapping));


When we always make shmem to set the mapping to enable large folios, can we then
drop this special casing no?

> +	VM_WARN_ON_ONCE(start & (HPAGE_PMD_NR - 1));
>  
>  	result = alloc_charge_folio(&new_folio, mm, cc, HPAGE_PMD_ORDER);
>  	if (result != SCAN_SUCCEED)


-- 
Cheers,

David