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 <rgold...@suse.com>
Signed-off-by: Shiyang Ruan <ruansy.f...@cn.fujitsu.com>
---
 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



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