mm/thp: allow dropping THP from page cache

Once a THP is added to the page cache, it cannot be dropped via
/proc/sys/vm/drop_caches.  Fix this issue with proper handling in
invalidate_mapping_pages().

Link: http://lkml.kernel.org/r/20191017164223.2762148-5-songliubraving@fb.com
Fixes: 99cb0db ("mm,thp: add read-only THP support for (non-shmem) FS")
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Song Liu <songliubraving@fb.com>
Tested-by: Song Liu <songliubraving@fb.com>
Acked-by: Yang Shi <yang.shi@linux.alibaba.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Cc: William Kucharski <william.kucharski@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/truncate.c

@@ -592,6 +592,16 @@ unsigned long invalidate_mapping_pages(struct address_space *mapping,
 					unlock_page(page);
 					continue;
 				}
+
+				/* Take a pin outside pagevec */
+				get_page(page);
+
+				/*
+				 * Drop extra pins before trying to invalidate
+				 * the huge page.
+				 */
+				pagevec_remove_exceptionals(&pvec);
+				pagevec_release(&pvec);
 			}
 
 			ret = invalidate_inode_page(page);
@@ -602,6 +612,8 @@ unsigned long invalidate_mapping_pages(struct address_space *mapping,
 			 */
 			if (!ret)
 				deactivate_file_page(page);
+			if (PageTransHuge(page))
+				put_page(page);
 			count += ret;
 		}
 		pagevec_remove_exceptionals(&pvec);