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

[Michal Hocko]

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 âfromâ. Is it better to split the THP into
> >> a list other than âfromâ and insert the list after âpageâ, then retry from the split âpageâ?
> >> 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 âfromâ 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@xxxxxxxxxxxxxx>

Thx!

-- 
Michal Hocko
SUSE Labs