Re: [PATCH] mm/vma: Do not register private-anon mappings with khugepaged during mmap

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Thu, Mar 06, 2025 at 12:00:37PM +0530, Dev Jain wrote:
> We already are registering private-anon VMAs with khugepaged during fault
> time, in do_huge_pmd_anonymous_page(). Commit "register suitable readonly
> file vmas for khugepaged" moved the khugepaged registration logic from
> shmem_mmap to the generic mmap path. Make this logic specific for non-anon
> mappings.

This does sound reasonable, thanks! Though does need to be expanded as
Andrew says for user-visible effects just to be crystal clear.

>
> Fixes: 613bec092fe7 ("mm: mmap: register suitable readonly file vmas for khugepaged")
> Signed-off-by: Dev Jain <dev.jain@arm.com>
> ---
>  mm/vma.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/mm/vma.c b/mm/vma.c
> index af1d549b179c..730a26bf14a5 100644
> --- a/mm/vma.c
> +++ b/mm/vma.c
> @@ -2377,7 +2377,8 @@ static int __mmap_new_vma(struct mmap_state *map, struct vm_area_struct **vmap)
>  	 * vma_merge_new_range() calls khugepaged_enter_vma() too, the below
>  	 * call covers the non-merge case.
>  	 */
> -	khugepaged_enter_vma(vma, map->flags);
> +	if (!vma_is_anonymous(vma))
> +		khugepaged_enter_vma(vma, map->flags);

This really needs a comment :) as a Joe Bloggs coder coming to this my
immediate thought would be 'huh? Why?'

Something like:

	/* Just added so khugepaged has nothing to do. We call again on fault. */

Would be great.

Thanks!


>  	ksm_add_vma(vma);
>  	*vmap = vma;
>  	return 0;
> --
> 2.30.2
>