[PATCH v4 1/2] iomap: Add a page_prepare callback

[Andreas Gruenbacher <agruenba@redhat.com>]

Move the page_done callback into a separate iomap_page_ops structure and
add a page_prepare calback to be called before a page is written to.  In
gfs2, we'll want to start a transaction in page_prepare and end it in
page_done, and other filesystems that implement data journaling will
require the same kind of mechanism.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
---
 fs/gfs2/bmap.c        | 22 +++++++++++++++++-----
 fs/iomap.c            | 22 ++++++++++++++++++----
 include/linux/iomap.h | 18 +++++++++++++-----
 3 files changed, 48 insertions(+), 14 deletions(-)

diff --git a/fs/gfs2/bmap.c b/fs/gfs2/bmap.c
index 5da4ca9041c0..281d739da652 100644
--- a/fs/gfs2/bmap.c
+++ b/fs/gfs2/bmap.c
@@ -991,15 +991,27 @@ 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,
-				unsigned copied, struct page *page,
-				struct iomap *iomap)
+static int gfs2_iomap_page_prepare(struct inode *inode, loff_t pos,
+				   unsigned len, struct iomap *iomap)
+{
+	return 0;
+}
+
+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);
 
-	gfs2_page_add_databufs(ip, page, offset_in_page(pos), copied);
+	if (page)
+		gfs2_page_add_databufs(ip, page, offset_in_page(pos), copied);
 }
 
+const struct iomap_page_ops gfs2_iomap_page_ops = {
+	.page_prepare = gfs2_iomap_page_prepare,
+	.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,
@@ -1077,7 +1089,7 @@ 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:
diff --git a/fs/iomap.c b/fs/iomap.c
index 97cb9d486a7d..667a822ecb7d 100644
--- a/fs/iomap.c
+++ b/fs/iomap.c
@@ -665,6 +665,7 @@ 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;
@@ -674,9 +675,17 @@ iomap_write_begin(struct inode *inode, loff_t pos, unsigned len, unsigned flags,
 	if (fatal_signal_pending(current))
 		return -EINTR;
 
+	if (page_ops) {
+		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 no_page;
+	}
 
 	if (iomap->type == IOMAP_INLINE)
 		iomap_read_inline_data(inode, page, iomap);
@@ -684,12 +693,16 @@ 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)) {
 		unlock_page(page);
 		put_page(page);
 		page = NULL;
 
 		iomap_write_failed(inode, pos, len);
+no_page:
+		if (page_ops)
+			page_ops->page_done(inode, pos, 0, NULL, iomap);
 	}
 
 	*pagep = page;
@@ -769,6 +782,7 @@ 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) {
@@ -780,8 +794,8 @@ iomap_write_end(struct inode *inode, loff_t pos, unsigned len,
 		ret = __iomap_write_end(inode, pos, len, copied, page, iomap);
 	}
 
-	if (iomap->page_done)
-		iomap->page_done(inode, pos, copied, page, iomap);
+	if (page_ops)
+		page_ops->page_done(inode, pos, copied, page, iomap);
 
 	if (ret < len)
 		iomap_write_failed(inode, pos, len);
diff --git a/include/linux/iomap.h b/include/linux/iomap.h
index 0fefb5455bda..fd65f27d300e 100644
--- a/include/linux/iomap.h
+++ b/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 +65,18 @@ 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.
-	 */
+/*
+ * Called before / after processing a page in the mapping returned in this
+ * iomap.  At least for now, this is only supported in the buffered write path.
+ * When page_prepare returns 0, page_done is called as well
+ * (possibly with page == NULL).
+ */
+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);
 };
-- 
2.20.1