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

[Dev Jain]

On 06/03/25 1:48 pm, Lorenzo Stoakes wrote:
> 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.

Sure.

> > Fixes: 613bec092fe7 ("mm: mmap: register suitable readonly file vmas for khugepaged")
> > Signed-off-by: Dev Jain <dev.jain@xxxxxxx>
> > ---
> >   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!

Sure.

BTW does this patch merit a CC:stable? I am not aware of the general 
practice but I am not sure if this is a *serious bug*, as per 
submitting-patches.rst.

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