mm: improve the comment in isolate_migratepages_block()

A recent patch shows that not everybody understands that "stabilise the
mapping" really means "prevent the mapping from being freed", so change
the wording to hopefully make that more clear.

Link: https://lkml.kernel.org/r/ZMLWEB4m3zvX6SBN@casper.infradead.orgSigned-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/compaction.c

@@ -1100,13 +1100,13 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
 			bool migrate_dirty;
 
 			/*
-			 * Only pages without mappings or that have a
-			 * ->migrate_folio callback are possible to migrate
-			 * without blocking. However, we can be racing with
-			 * truncation so it's necessary to lock the page
-			 * to stabilise the mapping as truncation holds
-			 * the page lock until after the page is removed
-			 * from the page cache.
+			 * Only folios without mappings or that have
+			 * a ->migrate_folio callback are possible to
+			 * migrate without blocking.  However, we may
+			 * be racing with truncation, which can free
+			 * the mapping.  Truncation holds the folio lock
+			 * until after the folio is removed from the page
+			 * cache so holding it ourselves is sufficient.
 			 */
 			if (!folio_trylock(folio))
 				goto isolate_fail_put;