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

Michal Hocko
2017-12-31T09:07:16Z

On Fri 29-12-17 10:45:46, Zi Yan wrote:
> On 29 Dec 2017, at 6:36, Michal Hocko wrote:
>
> > On Tue 26-12-17 21:19:35, Zi Yan wrote:
[...]
> >> 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?
>
> Functionally, it does not matter.
>
> This behavior is just less intuitive and a little different from current one,
> which implicitly preserves its original order of the not-migrated pages
> in the “from” list, although no one relies on this implicit behavior now.
>
>
> Adding one line comment about this difference would be good for code maintenance. :)

OK, I will not argue. I still do not see _why_ we need it but I've added
the following.

diff --git a/mm/migrate.c b/mm/migrate.c
index 21b3381a2871..0ac5185d3949 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1395,6 +1395,11 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
* allocation could've failed so we should
* retry on the same page with the THP split
* to base pages.
+ *
+ * Head page is retried immediatelly and tail
+ * pages are added to the tail of the list so
+ * we encounter them after the rest of the list
+ * is processed.
*/
if (PageTransHuge(page)) {
lock_page(page);

Does that this reflect what you mean?

> Reviewed-by: Zi Yan <zi.yan@cs.rutgers.edu>

Thx!

--
Michal Hocko
SUSE Labs