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

Lorenzo Stoakes (Oracle)
2026-03-27T12:07:24Z

On Thu, Mar 26, 2026 at 09:42:47PM -0400, 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 */

I think this isn't quite clear and could be improved to e.g.:

/*
* Either anon shmem supports huge pages as set by shmem_enabled sysfs,
* or a shmem file system mounted with the "huge" option.
*/

> + if (mapping_max_folio_order(mapping) < PMD_ORDER)
> + return SCAN_FAIL;

As per rest of thread, this looks correct.

> + if (start & (HPAGE_PMD_NR - 1))
> + return SCAN_ADDRESS_RANGE;

Hmm, we're kinda making this - presumably buggy situation - into a valid input
that just fails the scan.

Maybe just make it a VM_WARN_ON_ONCE()? Or if we want to avoid propagating the
bug that'd cause it any further:

if (start & (HPAGE_PMD_NR - 1)) {
VM_WARN_ON_ONCE(true);
return SCAN_ADDRESS_RANGE;
}

Or similar.

>
> result = alloc_charge_folio(&new_folio, mm, cc);
> if (result != SCAN_SUCCEED)
> --
> 2.43.0
>

Cheers, Lorenzo