Re: [RFC PATCH 3/3] mm: unclutter THP migration

Michal Hocko
2017-12-29T11:36:30Z

On Tue 26-12-17 21:19:35, Zi Yan wrote:
> On 8 Dec 2017, at 11:15, Michal Hocko wrote:
[...]
> > @@ -1394,6 +1390,21 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
> >
> > switch(rc) {
> > case -ENOMEM:
> > + /*
> > + * THP migration might be unsupported or the
> > + * allocation could've failed so we should
> > + * retry on the same page with the THP split
> > + * to base pages.
> > + */
> > + if (PageTransHuge(page)) {
> > + lock_page(page);
> > + rc = split_huge_page_to_list(page, from);
> > + unlock_page(page);
> > + if (!rc) {
> > + list_safe_reset_next(page, page2, lru);
> > + goto retry;
> > + }
> > + }
>
> The hunk splits the THP and adds all tail pages at the end of the list “from”.
> Why do we need “list_safe_reset_next(page, page2, lru);” here, when page2 is not changed here?

Because we need to handle the case when the page2 was the last on the
list.

> And it seems a little bit strange to only re-migrate the head page, then come back to all tail
> pages after migrating the rest of pages in the list “from”. Is it better to split the THP into
> a list other than “from” and insert the list after “page”, then retry from the split “page”?
> Thus, we attempt to migrate all sub pages of the THP after it is split.

Why does this matter?
--
Michal Hocko
SUSE Labs