[PATCH 2/7] fsdax: Introduce dax_copy_edges() for CoW

[Shiyang Ruan]

dax_copy_edges() is a helper functions performs a copy from one part of
the device to another for data not page aligned.

Signed-off-by: Goldwyn Rodrigues <rgoldwyn@xxxxxxxx>
Signed-off-by: Shiyang Ruan <ruansy.fnst@xxxxxxxxxxxxxx>
---
 fs/dax.c | 41 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 41 insertions(+)

diff --git a/fs/dax.c b/fs/dax.c
index b012b2db7ba2..ea4e8a434900 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -1038,6 +1038,47 @@ static int dax_iomap_direct_access(struct iomap *iomap, loff_t pos, size_t size,
 	return rc;
 }
 
+/*
+ * Copies the part of the pages not included in the write, but required for CoW
+ * because offset/offset+length are not page aligned.
+ */
+static int dax_copy_edges(loff_t pos, loff_t length, struct iomap *srcmap,
+			  void *daddr, bool pmd)
+{
+	size_t page_size = pmd ? PMD_SIZE : PAGE_SIZE;
+	loff_t offset = pos & (page_size - 1);
+	size_t size = ALIGN(offset + length, page_size);
+	loff_t end = pos + length;
+	loff_t pg_end = round_up(end, page_size);
+	void *saddr = 0;
+	int ret = 0;
+
+	ret = dax_iomap_direct_access(srcmap, pos, size, &saddr, NULL);
+	if (ret)
+		return ret;
+	/*
+	 * Copy the first part of the page
+	 * Note: we pass offset as length
+	 */
+	if (offset) {
+		if (saddr)
+			ret = copy_mc_to_kernel(daddr, saddr, offset);
+		else
+			memset(daddr, 0, offset);
+	}
+
+	/* Copy the last part of the range */
+	if (end < pg_end) {
+		if (saddr)
+			ret = copy_mc_to_kernel(daddr + offset + length,
+			       saddr + offset + length,	pg_end - end);
+		else
+			memset(daddr + offset + length, 0, pg_end - end);
+	}
+
+	return ret;
+}
+
 /*
  * The user has performed a load from a hole in the file.  Allocating a new
  * page in the file would cause excessive storage usage for workloads with
-- 
2.30.0