mm: migrate: fix THP's mapcount on isolation

The issue is reported when removing memory through virtio_mem device.  The
transparent huge page, experienced copy-on-write fault, is wrongly
regarded as pinned.  The transparent huge page is escaped from being
isolated in isolate_migratepages_block().  The transparent huge page can't
be migrated and the corresponding memory block can't be put into offline
state.

Fix it by replacing page_mapcount() with total_mapcount().  With this, the
transparent huge page can be isolated and migrated, and the memory block
can be put into offline state.  Besides, The page's refcount is increased
a bit earlier to avoid the page is released when the check is executed.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 1da2f32 ("mm,thp,compaction,cma: allow THP migration for CMA allocations")
Signed-off-by: Gavin Shan <[email protected]>
Reported-by: Zhenyu Zhang <[email protected]>
Tested-by: Zhenyu Zhang <[email protected]>
Suggested-by: David Hildenbrand <[email protected]>
Acked-by: David Hildenbrand <[email protected]>
Cc: Alistair Popple <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: William Kucharski <[email protected]>
Cc: Zi Yan <[email protected]>
Cc: <[email protected]>	[5.7+]
Signed-off-by: Andrew Morton <[email protected]>

Author: Gavin Shan

mm/compaction.c

@@ -984,29 +984,29 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
 			goto isolate_fail;
 		}
 
+		/*
+		 * Be careful not to clear PageLRU until after we're
+		 * sure the page is not being freed elsewhere -- the
+		 * page release code relies on it.
+		 */
+		if (unlikely(!get_page_unless_zero(page)))
+			goto isolate_fail;
+
 		/*
 		 * Migration will fail if an anonymous page is pinned in memory,
 		 * so avoid taking lru_lock and isolating it unnecessarily in an
 		 * admittedly racy check.
 		 */
 		mapping = page_mapping(page);
-		if (!mapping && page_count(page) > page_mapcount(page))
-			goto isolate_fail;
+		if (!mapping && (page_count(page) - 1) > total_mapcount(page))
+			goto isolate_fail_put;
 
 		/*
 		 * Only allow to migrate anonymous pages in GFP_NOFS context
 		 * because those do not depend on fs locks.
 		 */
 		if (!(cc->gfp_mask & __GFP_FS) && mapping)
-			goto isolate_fail;
-
-		/*
-		 * Be careful not to clear PageLRU until after we're
-		 * sure the page is not being freed elsewhere -- the
-		 * page release code relies on it.
-		 */
-		if (unlikely(!get_page_unless_zero(page)))
-			goto isolate_fail;
+			goto isolate_fail_put;
 
 		/* Only take pages on LRU: a check now makes later tests safe */
 		if (!PageLRU(page))