mpage: Provide variant of mpage_writepages() with own optional folio handler

Some filesystems need to treat some folios specially (for example for
inodes with inline data). Doing the handling in their .writepages method
in a race-free manner results in duplicating some of the writeback
internals. So provide generalized version of mpage_writepages() that
allows filesystem to provide a handler called for each folio which can
handle the folio in a special way.

Reviewed-by: Christoph Hellwig <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jan Kara <[email protected]>

Author: jankara

fs/mpage.c

@@ -646,17 +646,24 @@ static int mpage_write_folio(struct writeback_control *wbc, struct folio *folio,
 }
 
 /**
- * mpage_writepages - walk the list of dirty pages of the given address space & writepage() all of them
+ * __mpage_writepages - walk the list of dirty pages of the given address space
+ * 			& writepage() all of them
  * @mapping: address space structure to write
  * @wbc: subtract the number of written pages from *@wbc->nr_to_write
  * @get_block: the filesystem's block mapper function.
+ * @write_folio: handler to call for each folio before calling
+ *		 mpage_write_folio()
  *
  * This is a library function, which implements the writepages()
- * address_space_operation.
+ * address_space_operation. It calls @write_folio handler for each folio. If
+ * the handler returns value > 0, it calls mpage_write_folio() to do the
+ * folio writeback.
  */
 int
-mpage_writepages(struct address_space *mapping,
-		struct writeback_control *wbc, get_block_t get_block)
+__mpage_writepages(struct address_space *mapping,
+		   struct writeback_control *wbc, get_block_t get_block,
+		   int (*write_folio)(struct folio *folio,
+				      struct writeback_control *wbc))
 {
 	struct mpage_data mpd = {
 		.get_block	= get_block,
@@ -666,11 +673,22 @@ mpage_writepages(struct address_space *mapping,
 	int error;
 
 	blk_start_plug(&plug);
-	while ((folio = writeback_iter(mapping, wbc, folio, &error)))
+	while ((folio = writeback_iter(mapping, wbc, folio, &error))) {
+		if (write_folio) {
+			error = write_folio(folio, wbc);
+			/*
+			 * == 0 means folio is handled, < 0 means error. In
+			 * both cases hand back control to writeback_iter()
+			 */
+			if (error <= 0)
+				continue;
+			/* Let mpage_write_folio() handle the folio. */
+		}
 		error = mpage_write_folio(wbc, folio, &mpd);
+	}
 	if (mpd.bio)
 		mpage_bio_submit_write(mpd.bio);
 	blk_finish_plug(&plug);
 	return error;
 }
-EXPORT_SYMBOL(mpage_writepages);
+EXPORT_SYMBOL(__mpage_writepages);

include/linux/mpage.h

@@ -17,7 +17,14 @@ struct readahead_control;
 
 void mpage_readahead(struct readahead_control *, get_block_t get_block);
 int mpage_read_folio(struct folio *folio, get_block_t get_block);
-int mpage_writepages(struct address_space *mapping,
-		struct writeback_control *wbc, get_block_t get_block);
+int __mpage_writepages(struct address_space *mapping,
+		struct writeback_control *wbc, get_block_t get_block,
+		int (*write_folio)(struct folio *folio,
+				   struct writeback_control *wbc));
+static inline int mpage_writepages(struct address_space *mapping,
+		struct writeback_control *wbc, get_block_t get_block)
+{
+	return __mpage_writepages(mapping, wbc, get_block, NULL);
+}
 
 #endif