[PATCH -mm] mm/page_isolation: fix potential warning from user

[Qian Cai <cai@xxxxxx>]

It makes sense to call the WARN_ON_ONCE(zone_idx(zone) == ZONE_MOVABLE)
from the offlining path, but should avoid triggering it from userspace,
i.e, from is_mem_section_removable().

While at it, simplify the code a bit by removing an unnecessary jump
label and a local variable, so set_migratetype_isolate() could really
return a bool.

Suggested-by: Michal Hocko <mhocko@xxxxxxxxxx>
Signed-off-by: Qian Cai <cai@xxxxxx>
---
 mm/page_alloc.c     | 11 ++++-------
 mm/page_isolation.c | 31 ++++++++++++++++++-------------
 2 files changed, 22 insertions(+), 20 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 621716a25639..3c4eb750a199 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -8231,7 +8231,7 @@ struct page *has_unmovable_pages(struct zone *zone, struct page *page,
 		if (is_migrate_cma(migratetype))
 			return NULL;
 
-		goto unmovable;
+		return page;
 	}
 
 	for (; iter < pageblock_nr_pages; iter++) {
@@ -8241,7 +8241,7 @@ struct page *has_unmovable_pages(struct zone *zone, struct page *page,
 		page = pfn_to_page(pfn + iter);
 
 		if (PageReserved(page))
-			goto unmovable;
+			return page;
 
 		/*
 		 * If the zone is movable and we have ruled out all reserved
@@ -8261,7 +8261,7 @@ struct page *has_unmovable_pages(struct zone *zone, struct page *page,
 			unsigned int skip_pages;
 
 			if (!hugepage_migration_supported(page_hstate(head)))
-				goto unmovable;
+				return page;
 
 			skip_pages = compound_nr(head) - (page - head);
 			iter += skip_pages - 1;
@@ -8303,12 +8303,9 @@ struct page *has_unmovable_pages(struct zone *zone, struct page *page,
 		 * is set to both of a memory hole page and a _used_ kernel
 		 * page at boot.
 		 */
-		goto unmovable;
+		return page;
 	}
 	return NULL;
-unmovable:
-	WARN_ON_ONCE(zone_idx(zone) == ZONE_MOVABLE);
-	return pfn_to_page(pfn + iter);
 }
 
 #ifdef CONFIG_CONTIG_ALLOC
diff --git a/mm/page_isolation.c b/mm/page_isolation.c
index e70586523ca3..97f673d5fefa 100644
--- a/mm/page_isolation.c
+++ b/mm/page_isolation.c
@@ -15,12 +15,12 @@
 #define CREATE_TRACE_POINTS
 #include <trace/events/page_isolation.h>
 
-static int set_migratetype_isolate(struct page *page, int migratetype, int isol_flags)
+static bool set_migratetype_isolate(struct page *page, int migratetype,
+				    int isol_flags)
 {
-	struct page *unmovable = NULL;
+	struct page *unmovable = ERR_PTR(-EBUSY);
 	struct zone *zone;
 	unsigned long flags;
-	int ret = -EBUSY;
 
 	zone = page_zone(page);
 
@@ -49,21 +49,26 @@ static int set_migratetype_isolate(struct page *page, int migratetype, int isol_
 									NULL);
 
 		__mod_zone_freepage_state(zone, -nr_pages, mt);
-		ret = 0;
 	}
 
 out:
 	spin_unlock_irqrestore(&zone->lock, flags);
-	if (!ret)
+
+	if (!unmovable) {
 		drain_all_pages(zone);
-	else if ((isol_flags & REPORT_FAILURE) && unmovable)
-		/*
-		 * printk() with zone->lock held will guarantee to trigger a
-		 * lockdep splat, so defer it here.
-		 */
-		dump_page(unmovable, "unmovable page");
-
-	return ret;
+	} else {
+		if (isol_flags & MEMORY_OFFLINE)
+			WARN_ON_ONCE(zone_idx(zone) == ZONE_MOVABLE);
+
+		if ((isol_flags & REPORT_FAILURE) && !IS_ERR(unmovable))
+			/*
+			 * printk() with zone->lock held will likely trigger a
+			 * lockdep splat, so defer it here.
+			 */
+			dump_page(unmovable, "unmovable page");
+	}
+
+	return !!unmovable;
 }
 
 static void unset_migratetype_isolate(struct page *page, unsigned migratetype)
-- 
2.21.0 (Apple Git-122.2)