iomap: Add a page_prepare callback

Move the page_done callback into a separate iomap_page_ops structure and
add a page_prepare calback to be called before the next page is written
to.  In gfs2, we'll want to start a transaction in page_prepare and end
it in page_done.  Other filesystems that implement data journaling will
require the same kind of mechanism.
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

fs/gfs2/bmap.c

@@ -965,15 +965,20 @@ static void gfs2_write_unlock(struct inode *inode)
 	gfs2_glock_dq_uninit(&ip->i_gh);
 }
 
-static void gfs2_iomap_journaled_page_done(struct inode *inode, loff_t pos,
+static void gfs2_iomap_page_done(struct inode *inode, loff_t pos,
 				 unsigned copied, struct page *page,
 				 struct iomap *iomap)
 {
 	struct gfs2_inode *ip = GFS2_I(inode);
 
+	if (page)
 		gfs2_page_add_databufs(ip, page, offset_in_page(pos), copied);
 }
 
+static const struct iomap_page_ops gfs2_iomap_page_ops = {
+	.page_done = gfs2_iomap_page_done,
+};
+
 static int gfs2_iomap_begin_write(struct inode *inode, loff_t pos,
 				  loff_t length, unsigned flags,
 				  struct iomap *iomap,
@@ -1051,7 +1056,7 @@ static int gfs2_iomap_begin_write(struct inode *inode, loff_t pos,
 		}
 	}
 	if (!gfs2_is_stuffed(ip) && gfs2_is_jdata(ip))
-		iomap->page_done = gfs2_iomap_journaled_page_done;
+		iomap->page_ops = &gfs2_iomap_page_ops;
 	return 0;
 
 out_trans_end:

fs/iomap.c

@@ -657,6 +657,7 @@ static int
 iomap_write_begin(struct inode *inode, loff_t pos, unsigned len, unsigned flags,
 		struct page **pagep, struct iomap *iomap)
 {
+	const struct iomap_page_ops *page_ops = iomap->page_ops;
 	pgoff_t index = pos >> PAGE_SHIFT;
 	struct page *page;
 	int status = 0;
@@ -666,9 +667,17 @@ iomap_write_begin(struct inode *inode, loff_t pos, unsigned len, unsigned flags,
 	if (fatal_signal_pending(current))
 		return -EINTR;
 
+	if (page_ops && page_ops->page_prepare) {
+		status = page_ops->page_prepare(inode, pos, len, iomap);
+		if (status)
+			return status;
+	}
+
 	page = grab_cache_page_write_begin(inode->i_mapping, index, flags);
-	if (!page)
-		return -ENOMEM;
+	if (!page) {
+		status = -ENOMEM;
+		goto out_no_page;
+	}
 
 	if (iomap->type == IOMAP_INLINE)
 		iomap_read_inline_data(inode, page, iomap);
@@ -676,15 +685,21 @@ iomap_write_begin(struct inode *inode, loff_t pos, unsigned len, unsigned flags,
 		status = __block_write_begin_int(page, pos, len, NULL, iomap);
 	else
 		status = __iomap_write_begin(inode, pos, len, page, iomap);
-	if (unlikely(status)) {
+
+	if (unlikely(status))
+		goto out_unlock;
+
+	*pagep = page;
+	return 0;
+
+out_unlock:
 	unlock_page(page);
 	put_page(page);
-		page = NULL;
-
 	iomap_write_failed(inode, pos, len);
-	}
 
-	*pagep = page;
+out_no_page:
+	if (page_ops && page_ops->page_done)
+		page_ops->page_done(inode, pos, 0, NULL, iomap);
 	return status;
 }
 
@@ -758,6 +773,7 @@ static int
 iomap_write_end(struct inode *inode, loff_t pos, unsigned len,
 		unsigned copied, struct page *page, struct iomap *iomap)
 {
+	const struct iomap_page_ops *page_ops = iomap->page_ops;
 	int ret;
 
 	if (iomap->type == IOMAP_INLINE) {
@@ -770,8 +786,8 @@ iomap_write_end(struct inode *inode, loff_t pos, unsigned len,
 	}
 
 	__generic_write_end(inode, pos, ret, page);
-	if (iomap->page_done)
-		iomap->page_done(inode, pos, copied, page, iomap);
+	if (page_ops && page_ops->page_done)
+		page_ops->page_done(inode, pos, copied, page, iomap);
 	put_page(page);
 
 	if (ret < len)

include/linux/iomap.h

@@ -53,6 +53,8 @@ struct vm_fault;
  */
 #define IOMAP_NULL_ADDR -1ULL	/* addr is not valid */
 
+struct iomap_page_ops;
+
 struct iomap {
 	u64			addr; /* disk offset of mapping, bytes */
 	loff_t			offset;	/* file offset of mapping, bytes */
@@ -63,12 +65,22 @@ struct iomap {
 	struct dax_device	*dax_dev; /* dax_dev for dax operations */
 	void			*inline_data;
 	void			*private; /* filesystem private */
+	const struct iomap_page_ops *page_ops;
+};
 
-	/*
-	 * Called when finished processing a page in the mapping returned in
-	 * this iomap.  At least for now this is only supported in the buffered
-	 * write path.
+/*
+ * When a filesystem sets page_ops in an iomap mapping it returns, page_prepare
+ * and page_done will be called for each page written to.  This only applies to
+ * buffered writes as unbuffered writes will not typically have pages
+ * 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.
  */
+struct iomap_page_ops {
+	int (*page_prepare)(struct inode *inode, loff_t pos, unsigned len,
+			struct iomap *iomap);
 	void (*page_done)(struct inode *inode, loff_t pos, unsigned copied,
 			struct page *page, struct iomap *iomap);
 };