Re: [PATCH 5/6] mm: migrate: don't split THP for misplaced NUMA page

[Yang Shi <shy828301@gmail.com>]

On Tue, Mar 30, 2021 at 7:42 AM Gerald Schaefer
<gerald.schaefer@linux.ibm.com> wrote:
>
> On Mon, 29 Mar 2021 11:33:11 -0700
> Yang Shi <shy828301@gmail.com> wrote:
>
> > The old behavior didn't split THP if migration is failed due to lack of
> > memory on the target node.  But the THP migration does split THP, so keep
> > the old behavior for misplaced NUMA page migration.
> >
> > Signed-off-by: Yang Shi <shy828301@gmail.com>
> > ---
> >  mm/migrate.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/migrate.c b/mm/migrate.c
> > index 86325c750c14..1c0c873375ab 100644
> > --- a/mm/migrate.c
> > +++ b/mm/migrate.c
> > @@ -1444,6 +1444,7 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
> >       int swapwrite = current->flags & PF_SWAPWRITE;
> >       int rc, nr_subpages;
> >       LIST_HEAD(ret_pages);
> > +     bool nosplit = (reason == MR_NUMA_MISPLACED);
> >
> >       if (!swapwrite)
> >               current->flags |= PF_SWAPWRITE;
> > @@ -1495,7 +1496,7 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
> >                        */
> >                       case -ENOSYS:
> >                               /* THP migration is unsupported */
> > -                             if (is_thp) {
> > +                             if (is_thp && !nosplit) {
>
> This is the "THP migration is unsupported" case, but according to your
> description you rather want to change the -ENOMEM case?
>
> Could this be the correct place to trigger THP split for NUMA balancing,
> for architectures not supporting THP migration, like s390?

Yes, I think it could be as I mentioned in the previous email.

>
> Do I understand it correctly that this change (for -ENOSYS) would
> result in always failed THP migrations during NUMA balancing, if THP
> migration was not supported?

Yes.