[RFC PATCH 3/8] fs/dax: Introduce dax_copy_edges() for COW

[Shiyang Ruan <ruansy.fnst@xxxxxxxxxxxxxx>]

Add address output in dax_iomap_pfn() in order to perform a memcpy() in CoW
case.  Since this function both output address and pfn, rename it to
dax_iomap_direct_access().

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 | 64 ++++++++++++++++++++++++++++++++++++++++++++++++++------
 1 file changed, 58 insertions(+), 6 deletions(-)

diff --git a/fs/dax.c b/fs/dax.c
index 2f996c566103..8107ed10c851 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -1111,8 +1111,8 @@ static sector_t dax_iomap_sector(struct iomap *iomap, loff_t pos)
 	return (iomap->addr + (pos & PAGE_MASK) - iomap->offset) >> 9;
 }
 
-static int dax_iomap_pfn(struct iomap *iomap, loff_t pos, size_t size,
-			 pfn_t *pfnp)
+static int dax_iomap_direct_access(struct iomap *iomap, loff_t pos, size_t size,
+			 pfn_t *pfnp, void **addr)
 {
 	const sector_t sector = dax_iomap_sector(iomap, pos);
 	pgoff_t pgoff;
@@ -1123,12 +1123,14 @@ static int dax_iomap_pfn(struct iomap *iomap, loff_t pos, size_t size,
 	if (rc)
 		return rc;
 	id = dax_read_lock();
-	length = dax_direct_access(iomap->dax_dev, pgoff, PHYS_PFN(size),
-				   NULL, pfnp);
+	length = dax_direct_access(iomap->dax_dev, pgoff, PHYS_PFN(size), addr,
+				   pfnp);
 	if (length < 0) {
 		rc = length;
 		goto out;
 	}
+	if (!pfnp)
+		goto out_check_addr;
 	rc = -EINVAL;
 	if (PFN_PHYS(length) < size)
 		goto out;
@@ -1138,11 +1140,59 @@ static int dax_iomap_pfn(struct iomap *iomap, loff_t pos, size_t size,
 	if (length > 1 && !pfn_t_devmap(*pfnp))
 		goto out;
 	rc = 0;
+
+out_check_addr:
+	if (!addr)
+		goto out;
+	if (!*addr)
+		rc = -EFAULT;
 out:
 	dax_read_unlock(id);
 	return rc;
 }
 
+/*
+ * dax_copy_edges - 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, NULL, &saddr);
+	if (ret)
+		return ret;
+	/*
+	 * Copy the first part of the page
+	 * Note: we pass offset as length
+	 */
+	if (offset) {
+		if (saddr)
+			ret = memcpy_mcsafe(daddr, saddr, offset);
+		else
+			memset(daddr, 0, offset);
+	}
+
+	/* Copy the last part of the range */
+	if (end < pg_end) {
+		if (saddr)
+			ret = memcpy_mcsafe(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
@@ -1462,7 +1512,8 @@ static vm_fault_t dax_iomap_pte_fault(struct vm_fault *vmf, pfn_t *pfnp,
 			count_memcg_event_mm(vma->vm_mm, PGMAJFAULT);
 			major = VM_FAULT_MAJOR;
 		}
-		error = dax_iomap_pfn(&iomap, pos, PAGE_SIZE, &pfn);
+		error = dax_iomap_direct_access(&iomap, pos, PAGE_SIZE, &pfn,
+						NULL);
 		if (error < 0)
 			goto error_finish_iomap;
 
@@ -1680,7 +1731,8 @@ static vm_fault_t dax_iomap_pmd_fault(struct vm_fault *vmf, pfn_t *pfnp,
 
 	switch (iomap.type) {
 	case IOMAP_MAPPED:
-		error = dax_iomap_pfn(&iomap, pos, PMD_SIZE, &pfn);
+		error = dax_iomap_direct_access(&iomap, pos, PMD_SIZE, &pfn,
+						NULL);
 		if (error < 0)
 			goto finish_iomap;
 
-- 
2.26.2