[patch] mm, page_alloc: move_freepages should not examine struct page of reserved memory

[David Rientjes <rientjes@google.com>]

After commit 907ec5fca3dc ("mm: zero remaining unavailable struct pages"),
struct page of reserved memory is zeroed.  This causes page->flags to be 0
and fixes issues related to reading /proc/kpageflags, for example, of
reserved memory.

The VM_BUG_ON() in move_freepages_block(), however, assumes that
page_zone() is meaningful even for reserved memory.  That assumption is no
longer true after the aforementioned commit.

There's no reason why move_freepages_block() should be testing the
legitimacy of page_zone() for reserved memory; its scope is limited only
to pages on the zone's freelist.

Note that pfn_valid() can be true for reserved memory: there is a backing
struct page.  The check for page_to_nid(page) is also buggy but reserved
memory normally only appears on node 0 so the zeroing doesn't affect this.

Move the debug checks to after verifying PageBuddy is true.  This isolates
the scope of the checks to only be for buddy pages which are on the zone's
freelist which move_freepages_block() is operating on.  In this case, an
incorrect node or zone is a bug worthy of being warned about (and the
examination of struct page is acceptable bcause this memory is not
reserved).

Signed-off-by: David Rientjes <rientjes@google.com>
---
 mm/page_alloc.c | 19 ++++---------------
 1 file changed, 4 insertions(+), 15 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2238,27 +2238,12 @@ static int move_freepages(struct zone *zone,
 	unsigned int order;
 	int pages_moved = 0;
 
-#ifndef CONFIG_HOLES_IN_ZONE
-	/*
-	 * page_zone is not safe to call in this context when
-	 * CONFIG_HOLES_IN_ZONE is set. This bug check is probably redundant
-	 * anyway as we check zone boundaries in move_freepages_block().
-	 * Remove at a later date when no bug reports exist related to
-	 * grouping pages by mobility
-	 */
-	VM_BUG_ON(pfn_valid(page_to_pfn(start_page)) &&
-	          pfn_valid(page_to_pfn(end_page)) &&
-	          page_zone(start_page) != page_zone(end_page));
-#endif
 	for (page = start_page; page <= end_page;) {
 		if (!pfn_valid_within(page_to_pfn(page))) {
 			page++;
 			continue;
 		}
 
-		/* Make sure we are not inadvertently changing nodes */
-		VM_BUG_ON_PAGE(page_to_nid(page) != zone_to_nid(zone), page);
-
 		if (!PageBuddy(page)) {
 			/*
 			 * We assume that pages that could be isolated for
@@ -2273,6 +2258,10 @@ static int move_freepages(struct zone *zone,
 			continue;
 		}
 
+		/* Make sure we are not inadvertently changing nodes */
+		VM_BUG_ON_PAGE(page_to_nid(page) != zone_to_nid(zone), page);
+		VM_BUG_ON_PAGE(page_zone(page) != zone, page);
+
 		order = page_order(page);
 		move_to_free_area(page, &zone->free_area[order], migratetype);
 		page += 1 << order;