Re: [PATCH v1 02/10] mm/khugepaged: remove READ_ONLY_THP_FOR_FS check

Baolin Wang
2026-03-27T09:44:59Z

On 3/27/26 9:42 AM, 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. shmem with
> huge option turned on also sets large folio order on mapping, so the check
> also applies to shmem.
>
> While at it, replace VM_BUG_ON with returning failure values.
>
> Signed-off-by: Zi Yan <ziy@nvidia.com>
> ---
> mm/khugepaged.c | 7 +++++--
> 1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index d06d84219e1b..45b12ffb1550 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1899,8 +1899,11 @@ 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));
> + /* "huge" shmem sets mapping folio order and passes the check below */
> + if (mapping_max_folio_order(mapping) < PMD_ORDER)
> + return SCAN_FAIL;

This is not true for anonymous shmem, since its large order allocation
logic is similar to anonymous memory. That means it will not call
mapping_set_large_folios() for anonymous shmem.

So I think the check should be:

if (!is_shmem && mapping_max_folio_order(mapping) < PMD_ORDER)
return SCAN_FAIL;