Re: [PATCH v5 01/14] mm/khugepaged: remove READ_ONLY_THP_FOR_FS check

[Zi Yan]

On 29 Apr 2026, at 11:29, 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_folio_support() for FSes supporting large
> folio with at least PMD_ORDER.
>
> Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
> Reviewed-by: Lance Yang <lance.yang@xxxxxxxxx>
> Reviewed-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> ---
>  include/linux/pagemap.h | 26 ++++++++++++++++++++++++++
>  mm/khugepaged.c         | 10 ++++++++--
>  2 files changed, 34 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index 1f50991b43e3b..1fed3414fe9b8 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -513,6 +513,32 @@ static inline bool mapping_large_folio_support(const struct address_space *mappi
>  	return mapping_max_folio_order(mapping) > 0;
>  }
>
> +/**
> + * mapping_pmd_folio_support() - Check if a mapping support PMD-sized folio
> + * @mapping: The address_space
> + *
> + * Some file supports large folio but does not support as large as PMD order.
> + * If a PMD-sized pagecache folio is attempted to be created on a filesystem,
> + * this check needs to be performed first.
> + *
> + * Return: true - PMD-sized folio is supported, false - PMD-sized folio is not
> + * supported.
> + */
> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +static inline bool mapping_pmd_folio_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;
> +}
> +#else
> +static inline bool mapping_pmd_folio_support(const struct address_space *mapping)
> +{
> +	return false;
> +}
> +#endif
> +
>  /* 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 e112525c4aa9c..6808f2b48d864 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_folio_support(mapping));

sashiko asked:
Is it possible for userspace to intentionally trigger this warning via
madvise(MADV_COLLAPSE) on an unsupported read-only file? Although in
a later commit, modified file_thp_enabled() will prevent this.

Answer:
It is possible when CONFIG_READ_ONLY_THP_FOR_FS is enabled, but it is
unlikely a kernel will be shipped at this exact commit.


> +	VM_WARN_ON_ONCE(start & (HPAGE_PMD_NR - 1));
>
>  	result = alloc_charge_folio(&new_folio, mm, cc, HPAGE_PMD_ORDER);
>  	if (result != SCAN_SUCCEED)
> -- 
> 2.53.0


Best Regards,
Yan, Zi