[PATCH v6 18/22] xip: Add xip_zero_page_range

[Matthew Wilcox <matthew.r.wilcox@intel.com>]

This new function allows us to support hole-punch for XIP files by zeroing
a partial page, as opposed to the xip_truncate_page() function which can
only truncate to the end of the page.  Reimplement xip_truncate_page() as
a macro that calls xip_zero_page_range().

Signed-off-by: Matthew Wilcox <matthew.r.wilcox@intel.com>
[ported to 3.13-rc2]
Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
---
 Documentation/filesystems/dax.txt |  1 +
 fs/dax.c                          | 22 +++++++++++++++-------
 include/linux/fs.h                |  9 ++++++++-
 3 files changed, 24 insertions(+), 8 deletions(-)

diff --git a/Documentation/filesystems/dax.txt b/Documentation/filesystems/dax.txt
index 06f84e5..e5706cc 100644
--- a/Documentation/filesystems/dax.txt
+++ b/Documentation/filesystems/dax.txt
@@ -55,6 +55,7 @@ Filesystem support consists of
   for fault and page_mkwrite (which should probably call dax_fault() and
   dax_mkwrite(), passing the appropriate get_block() callback)
 - calling dax_truncate_page() instead of block_truncate_page() for DAX files
+- calling dax_zero_page_range() instead of zero_user() for DAX files
 - ensuring that there is sufficient locking between reads, writes,
   truncates and page faults
 
diff --git a/fs/dax.c b/fs/dax.c
index 75328bf..cdc8012 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -393,13 +393,16 @@ int dax_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf,
 EXPORT_SYMBOL_GPL(dax_mkwrite);
 
 /**
- * dax_truncate_page - handle a partial page being truncated in a DAX file
+ * dax_zero_page_range - zero a range within a page of a DAX file
  * @inode: The file being truncated
  * @from: The file offset that is being truncated to
+ * @length: The number of bytes to zero
  * @get_block: The filesystem method used to translate file offsets to blocks
  *
- * Similar to block_truncate_page(), this function can be called by a
- * filesystem when it is truncating an DAX file to handle the partial page.
+ * This function can be called by a filesystem when it is zeroing part of a
+ * page in a DAX file.  This is intended for hole-punch operations.  If
+ * you are truncating a file, the helper function dax_truncate_page() may be
+ * more convenient.
  *
  * We work in terms of PAGE_CACHE_SIZE here for commonality with
  * block_truncate_page(), but we could go down to PAGE_SIZE if the filesystem
@@ -407,12 +410,12 @@ EXPORT_SYMBOL_GPL(dax_mkwrite);
  * block size is smaller than PAGE_SIZE, we have to zero the rest of the page
  * since the file might be mmaped.
  */
-int dax_truncate_page(struct inode *inode, loff_t from, get_block_t get_block)
+int dax_zero_page_range(struct inode *inode, loff_t from, unsigned length,
+							get_block_t get_block)
 {
 	struct buffer_head bh;
 	pgoff_t index = from >> PAGE_CACHE_SHIFT;
 	unsigned offset = from & (PAGE_CACHE_SIZE-1);
-	unsigned length = PAGE_CACHE_ALIGN(from) - from;
 	int err;
 
 	/* Block boundary? Nothing to do */
@@ -427,11 +430,16 @@ int dax_truncate_page(struct inode *inode, loff_t from, get_block_t get_block)
 	if (buffer_written(&bh)) {
 		void *addr;
 		err = dax_get_addr(inode, &bh, &addr);
-		if (err)
+		if (err < 0)
 			return err;
+		/*
+		 * ext4 sometimes asks to zero past the end of a block.  It
+		 * really just wants to zero to the end of the block.
+		 */
+		length = min_t(unsigned, length, PAGE_CACHE_SIZE - offset);
 		memset(addr + offset, 0, length);
 	}
 
 	return 0;
 }
-EXPORT_SYMBOL_GPL(dax_truncate_page);
+EXPORT_SYMBOL_GPL(dax_zero_page_range);
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 19abdb1..6a5091a 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2517,6 +2517,7 @@ extern int nonseekable_open(struct inode * inode, struct file * filp);
 
 #ifdef CONFIG_FS_DAX
 int dax_clear_blocks(struct inode *, sector_t block, long size);
+int dax_zero_page_range(struct inode *, loff_t from, unsigned len, get_block_t);
 int dax_truncate_page(struct inode *, loff_t from, get_block_t);
 ssize_t dax_do_io(int rw, struct kiocb *, struct inode *, const struct iovec *,
 		loff_t, unsigned segs, get_block_t, dio_iodone_t, int flags);
@@ -2528,7 +2529,8 @@ static inline int dax_clear_blocks(struct inode *i, sector_t blk, long sz)
 	return 0;
 }
 
-static inline int dax_truncate_page(struct inode *i, loff_t frm, get_block_t gb)
+static inline int dax_zero_page_range(struct inode *inode, loff_t from,
+						unsigned len, get_block_t gb)
 {
 	return 0;
 }
@@ -2541,6 +2543,11 @@ static inline ssize_t dax_do_io(int rw, struct kiocb *iocb, struct inode *inode,
 }
 #endif
 
+/* Can't be a function because PAGE_CACHE_SIZE is defined in pagemap.h */
+#define dax_truncate_page(inode, from, get_block)	\
+	dax_zero_page_range(inode, from, PAGE_CACHE_SIZE, get_block)
+
+
 #ifdef CONFIG_BLOCK
 typedef void (dio_submit_t)(int rw, struct bio *bio, struct inode *inode,
 			    loff_t file_offset);
-- 
1.8.5.3