[RFC v6 02/10] iomap/gfs2: Unlock and put folio in page_done handler

[Andreas Gruenbacher <agruenba@redhat.com>]

When an iomap defines a ->page_done() handler in its page_ops, delegate
unlocking the folio and putting the folio reference to that handler.

This allows to fix a race between journaled data writes and folio
writeback in gfs2: before this change, gfs2_iomap_page_done() was called
after unlocking the folio, so writeback could start writing back the
folio's buffers before they could be marked for writing to the journal.
Also, try_to_free_buffers() could free the buffers before
gfs2_iomap_page_done() was done adding the buffers to the current
current transaction.  With this change, gfs2_iomap_page_done() adds the
buffers to the current transaction while the folio is still locked, so
the problems described above can no longer occur.

The only current user of ->page_done() is gfs2, so other filesystems are
not affected.  To catch out any out-of-tree users, switch from a page to
a folio in ->page_done().

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
---
 fs/gfs2/bmap.c         | 15 ++++++++++++---
 fs/iomap/buffered-io.c |  8 ++++----
 include/linux/iomap.h  |  7 ++++---
 3 files changed, 20 insertions(+), 10 deletions(-)

diff --git a/fs/gfs2/bmap.c b/fs/gfs2/bmap.c
index e7537fd305dd..46206286ad42 100644
--- a/fs/gfs2/bmap.c
+++ b/fs/gfs2/bmap.c
@@ -968,14 +968,23 @@ static int gfs2_iomap_page_prepare(struct inode *inode, loff_t pos,
 }
 
 static void gfs2_iomap_page_done(struct inode *inode, loff_t pos,
-				 unsigned copied, struct page *page)
+				 unsigned copied, struct folio *folio)
 {
 	struct gfs2_trans *tr = current->journal_info;
 	struct gfs2_inode *ip = GFS2_I(inode);
 	struct gfs2_sbd *sdp = GFS2_SB(inode);
 
-	if (page && !gfs2_is_stuffed(ip))
-		gfs2_page_add_databufs(ip, page, offset_in_page(pos), copied);
+	if (!folio) {
+		gfs2_trans_end(sdp);
+		return;
+	}
+
+	if (!gfs2_is_stuffed(ip))
+		gfs2_page_add_databufs(ip, &folio->page, offset_in_page(pos),
+				       copied);
+
+	folio_unlock(folio);
+	folio_put(folio);
 
 	if (tr->tr_num_buf_new)
 		__mark_inode_dirty(inode, I_DIRTY_DATASYNC);
diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
index c045689b6af8..a9082078e4ed 100644
--- a/fs/iomap/buffered-io.c
+++ b/fs/iomap/buffered-io.c
@@ -580,12 +580,12 @@ static void __iomap_put_folio(struct iomap_iter *iter, loff_t pos, size_t ret,
 {
 	const struct iomap_page_ops *page_ops = iter->iomap.page_ops;
 
-	if (folio)
+	if (page_ops && page_ops->page_done) {
+		page_ops->page_done(iter->inode, pos, ret, folio);
+	} else if (folio) {
 		folio_unlock(folio);
-	if (page_ops && page_ops->page_done)
-		page_ops->page_done(iter->inode, pos, ret, &folio->page);
-	if (folio)
 		folio_put(folio);
+	}
 }
 
 static int iomap_write_begin_inline(const struct iomap_iter *iter,
diff --git a/include/linux/iomap.h b/include/linux/iomap.h
index 0983dfc9a203..743e2a909162 100644
--- a/include/linux/iomap.h
+++ b/include/linux/iomap.h
@@ -131,13 +131,14 @@ static inline bool iomap_inline_data_valid(const struct iomap *iomap)
  * associated with them.
  *
  * When page_prepare succeeds, page_done will always be called to do any
- * cleanup work necessary.  In that page_done call, @page will be NULL if the
- * associated page could not be obtained.
+ * cleanup work necessary.  In that page_done call, @folio will be NULL if the
+ * associated folio could not be obtained.  When folio is not NULL, page_done
+ * is responsible for unlocking and putting the folio.
  */
 struct iomap_page_ops {
 	int (*page_prepare)(struct inode *inode, loff_t pos, unsigned len);
 	void (*page_done)(struct inode *inode, loff_t pos, unsigned copied,
-			struct page *page);
+			struct folio *folio);
 
 	/*
 	 * Check that the cached iomap still maps correctly to the filesystem's
-- 
2.38.1