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

["Lorenzo Stoakes (Oracle)" <ljs@kernel.org>]

On Fri, Mar 27, 2026 at 05:44:49PM +0800, Baolin Wang wrote:
>
>
> 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;

Hmm but in shmem_init() we have:

#ifdef CONFIG_TRANSPARENT_HUGEPAGE
	if (has_transparent_hugepage() && shmem_huge > SHMEM_HUGE_DENY)
		SHMEM_SB(shm_mnt->mnt_sb)->huge = shmem_huge;
	else
		shmem_huge = SHMEM_HUGE_NEVER; /* just in case it was patched */

	/*
	 * Default to setting PMD-sized THP to inherit the global setting and
	 * disable all other multi-size THPs.
	 */
	if (!shmem_orders_configured)
		huge_shmem_orders_inherit = BIT(HPAGE_PMD_ORDER);
#endif

And shm_mnt->mnt_sb is the superblock used for anon shmem. Also
shmem_enabled_store() updates that if necessary.

So we're still fine right?

__shmem_file_setup() (used for anon shmem) calls shmem_get_inode() ->
__shmem_get_inode() which has:

	if (sbinfo->huge)
		mapping_set_large_folios(inode->i_mapping);

Shared for both anon shmem and tmpfs-style shmem.

So I think it's fine as-is.

Cheers, Lorenzo