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

Zi Yan
2026-04-30T14:38:04Z

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@nvidia.com>
> Reviewed-by: Lance Yang <lance.yang@linux.dev>
> Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> ---
> 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