mm: unclutter THP migration

THP migration is hacked into the generic migration with rather
surprising semantic.  The migration allocation callback is supposed to
check whether the THP can be migrated at once and if that is not the
case then it allocates a simple page to migrate.  unmap_and_move then
fixes that up by spliting the THP into small pages while moving the head
page to the newly allocated order-0 page.  Remaning pages are moved to
the LRU list by split_huge_page.  The same happens if the THP allocation
fails.  This is really ugly and error prone [1].

I also believe that split_huge_page to the LRU lists is inherently wrong
because all tail pages are not migrated.  Some callers will just work
around that by retrying (e.g.  memory hotplug).  There are other pfn
walkers which are simply broken though.  e.g. madvise_inject_error will
migrate head and then advances next pfn by the huge page size.
do_move_page_to_node_array, queue_pages_range (migrate_pages, mbind),
will simply split the THP before migration if the THP migration is not
supported then falls back to single page migration but it doesn't handle
tail pages if the THP migration path is not able to allocate a fresh THP
so we end up with ENOMEM and fail the whole migration which is a
questionable behavior.  Page compaction doesn't try to migrate large
pages so it should be immune.

This patch tries to unclutter the situation by moving the special THP
handling up to the migrate_pages layer where it actually belongs.  We
simply split the THP page into the existing list if unmap_and_move fails
with ENOMEM and retry.  So we will _always_ migrate all THP subpages and
specific migrate_pages users do not have to deal with this case in a
special way.

[1] http://lkml.kernel.org/r/20171121021855.50525-1-zi.yan@sent.com

Link: http://lkml.kernel.org/r/20180103082555.14592-4-mhocko@kernel.org
Signed-off-by: Michal Hocko <mhocko@suse.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reviewed-by: Zi Yan <zi.yan@cs.rutgers.edu>
Cc: Andrea Reale <ar@linux.vnet.ibm.com>
Cc: Anshuman Khandual <khandual@linux.vnet.ibm.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 24 insertions, 10 deletions

diff --git a/mm/migrate.c b/mm/migrate.c
index c606752f6d2a..51b55f2d2db5 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1139,6 +1139,9 @@ static ICE_noinline int unmap_and_move(new_page_t get_new_page,
 	int rc = MIGRATEPAGE_SUCCESS;
 	struct page *newpage;
 
+	if (!thp_migration_supported() && PageTransHuge(page))
+		return -ENOMEM;
+
 	newpage = get_new_page(page, private);
 	if (!newpage)
 		return -ENOMEM;
@@ -1160,14 +1163,6 @@ static ICE_noinline int unmap_and_move(new_page_t get_new_page,
 		goto out;
 	}
 
-	if (unlikely(PageTransHuge(page) && !PageTransHuge(newpage))) {
-		lock_page(page);
-		rc = split_huge_page(page);
-		unlock_page(page);
-		if (rc)
-			goto out;
-	}
-
 	rc = __unmap_and_move(page, newpage, force, mode);
 	if (rc == MIGRATEPAGE_SUCCESS)
 		set_page_owner_migrate_reason(newpage, reason);
@@ -1381,6 +1376,7 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
 		retry = 0;
 
 		list_for_each_entry_safe(page, page2, from, lru) {
+retry:
 			cond_resched();
 
 			if (PageHuge(page))
@@ -1394,6 +1390,26 @@ 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.
+				 *
+				 * Head page is retried immediately 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);
+					rc = split_huge_page_to_list(page, from);
+					unlock_page(page);
+					if (!rc) {
+						list_safe_reset_next(page, page2, lru);
+						goto retry;
+					}
+				}
 				nr_failed++;
 				goto out;
 			case -EAGAIN:
@@ -1480,8 +1496,6 @@ static int add_page_for_migration(struct mm_struct *mm, unsigned long addr,
 
 	/* FOLL_DUMP to ignore special (like zero) pages */
 	follflags = FOLL_GET | FOLL_DUMP;
-	if (!thp_migration_supported())
-		follflags |= FOLL_SPLIT;
 	page = follow_page(vma, addr, follflags);
 
 	err = PTR_ERR(page);