mm/filemap: Add folio_wait_locked()

Also add folio_wait_locked_killable().  Turn wait_on_page_locked() and
wait_on_page_locked_killable() into wrappers.  This eliminates a call
to compound_head() from each call-site, reducing text size by 193 bytes
for me.
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Jeff Layton <jlayton@kernel.org>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: William Kucharski <william.kucharski@oracle.com>
Reviewed-by: David Howells <dhowells@redhat.com>
Acked-by: Mike Rapoport <rppt@linux.ibm.com>

include/linux/pagemap.h

@@ -732,23 +732,33 @@ extern void wait_on_page_bit(struct page *page, int bit_nr);
 extern int wait_on_page_bit_killable(struct page *page, int bit_nr);
 
 /* 
- * Wait for a page to be unlocked.
+ * Wait for a folio to be unlocked.
  *
- * This must be called with the caller "holding" the page,
- * ie with increased "page->count" so that the page won't
+ * This must be called with the caller "holding" the folio,
+ * ie with increased "page->count" so that the folio won't
  * go away during the wait..
  */
+static inline void folio_wait_locked(struct folio *folio)
+{
+	if (folio_test_locked(folio))
+		wait_on_page_bit(&folio->page, PG_locked);
+}
+
+static inline int folio_wait_locked_killable(struct folio *folio)
+{
+	if (!folio_test_locked(folio))
+		return 0;
+	return wait_on_page_bit_killable(&folio->page, PG_locked);
+}
+
 static inline void wait_on_page_locked(struct page *page)
 {
-	if (PageLocked(page))
-		wait_on_page_bit(compound_head(page), PG_locked);
+	folio_wait_locked(page_folio(page));
 }
 
 static inline int wait_on_page_locked_killable(struct page *page)
 {
-	if (!PageLocked(page))
-		return 0;
-	return wait_on_page_bit_killable(compound_head(page), PG_locked);
+	return folio_wait_locked_killable(page_folio(page));
 }
 
 int put_and_wait_on_page_locked(struct page *page, int state);

mm/filemap.c

@@ -1704,9 +1704,9 @@ int __lock_page_or_retry(struct page *page, struct mm_struct *mm,
 
 		mmap_read_unlock(mm);
 		if (flags & FAULT_FLAG_KILLABLE)
-			wait_on_page_locked_killable(page);
+			folio_wait_locked_killable(folio);
 		else
-			wait_on_page_locked(page);
+			folio_wait_locked(folio);
 		return 0;
 	}
 	if (flags & FAULT_FLAG_KILLABLE) {