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

David Hildenbrand (Arm)
2026-04-24T12:40:29Z

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