mm: page migration avoid touching newpage until no going back
We have had trouble in the past from the way in which page migration's
newpage is initialized in dribs and drabs - see commit 8bdd63809160 ("mm:
fix direct reclaim writeback regression") which proposed a cleanup.
We have no actual problem now, but I think the procedure would be clearer
(and alternative get_new_page pools safer to implement) if we assert that
newpage is not touched until we are sure that it's going to be used -
except for taking the trylock on it in __unmap_and_move().
So shift the early initializations from move_to_new_page() into
migrate_page_move_mapping(), mapping and NULL-mapping paths. Similarly
migrate_huge_page_move_mapping(), but its NULL-mapping path can just be
deleted: you cannot reach hugetlbfs_migrate_page() with a NULL mapping.
Adjust stages 3 to 8 in the Documentation file accordingly.
Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Sasha Levin <sasha.levin@oracle.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/mm/migrate.c b/mm/migrate.c
index 08a7b6c..3067e40 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -320,6 +320,14 @@
/* Anonymous page without mapping */
if (page_count(page) != expected_count)
return -EAGAIN;
+
+ /* No turning back from here */
+ set_page_memcg(newpage, page_memcg(page));
+ newpage->index = page->index;
+ newpage->mapping = page->mapping;
+ if (PageSwapBacked(page))
+ SetPageSwapBacked(newpage);
+
return MIGRATEPAGE_SUCCESS;
}
@@ -355,8 +363,15 @@
}
/*
- * Now we know that no one else is looking at the page.
+ * Now we know that no one else is looking at the page:
+ * no turning back from here.
*/
+ set_page_memcg(newpage, page_memcg(page));
+ newpage->index = page->index;
+ newpage->mapping = page->mapping;
+ if (PageSwapBacked(page))
+ SetPageSwapBacked(newpage);
+
get_page(newpage); /* add cache reference */
if (PageSwapCache(page)) {
SetPageSwapCache(newpage);
@@ -403,12 +418,6 @@
int expected_count;
void **pslot;
- if (!mapping) {
- if (page_count(page) != 1)
- return -EAGAIN;
- return MIGRATEPAGE_SUCCESS;
- }
-
spin_lock_irq(&mapping->tree_lock);
pslot = radix_tree_lookup_slot(&mapping->page_tree,
@@ -426,6 +435,9 @@
return -EAGAIN;
}
+ set_page_memcg(newpage, page_memcg(page));
+ newpage->index = page->index;
+ newpage->mapping = page->mapping;
get_page(newpage);
radix_tree_replace_slot(pslot, newpage);
@@ -730,21 +742,6 @@
VM_BUG_ON_PAGE(!PageLocked(page), page);
VM_BUG_ON_PAGE(!PageLocked(newpage), newpage);
- /* Prepare mapping for the new page.*/
- newpage->index = page->index;
- newpage->mapping = page->mapping;
- if (PageSwapBacked(page))
- SetPageSwapBacked(newpage);
-
- /*
- * Indirectly called below, migrate_page_copy() copies PG_dirty and thus
- * needs newpage's memcg set to transfer memcg dirty page accounting.
- * So perform memcg migration in two steps:
- * 1. set newpage->mem_cgroup (here)
- * 2. clear page->mem_cgroup (below)
- */
- set_page_memcg(newpage, page_memcg(page));
-
mapping = page_mapping(page);
if (!mapping)
rc = migrate_page(mapping, newpage, page, mode);
@@ -767,9 +764,6 @@
set_page_memcg(page, NULL);
if (!PageAnon(page))
page->mapping = NULL;
- } else {
- set_page_memcg(newpage, NULL);
- newpage->mapping = NULL;
}
return rc;
}
@@ -971,10 +965,9 @@
* it. Otherwise, putback_lru_page() will drop the reference grabbed
* during isolation.
*/
- if (put_new_page) {
- ClearPageSwapBacked(newpage);
+ if (put_new_page)
put_new_page(newpage, private);
- } else if (unlikely(__is_movable_balloon_page(newpage))) {
+ else if (unlikely(__is_movable_balloon_page(newpage))) {
/* drop our reference, page already in the balloon */
put_page(newpage);
} else