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

[Lance Yang <lance.yang@linux.dev>]

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

Yep, for shmem inodes, if the mount has huge= enabled, inode creation
marks the mapping are large-folio capable:

	/* Don't consider 'deny' for emergencies and 'force' for testing */
	if (sbinfo->huge)
		mapping_set_large_folios(inode->i_mapping);

LGTM!

Reviewed-by: Lance Yang <lance.yang@linux.dev>