dax: add dax_zero_page_range

This new function allows us to support hole-punch for DAX files by zeroing
a partial page, as opposed to the dax_truncate_page() function which can
only truncate to the end of the page.  Reimplement dax_truncate_page() to
call dax_zero_page_range().

[ross.zwisler@linux.intel.com: ported to 3.13-rc2]
[akpm@linux-foundation.org: fix typos in comments]
Signed-off-by: Matthew Wilcox <matthew.r.wilcox@intel.com>
Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Cc: Andreas Dilger <andreas.dilger@intel.com>
Cc: Boaz Harrosh <boaz@plexistor.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 32 insertions, 6 deletions

diff --git a/fs/dax.c b/fs/dax.c
index ebf9b2231b0f..ed1619ec6537 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -464,31 +464,35 @@ int dax_fault(struct vm_area_struct *vma, struct vm_fault *vmf,
 EXPORT_SYMBOL_GPL(dax_fault);
 
 /**
- * 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
  * took care of disposing of the unnecessary blocks.  Even if the filesystem
  * block size is smaller than PAGE_SIZE, we have to zero the rest of the page
- * since the file might be mmaped.
+ * since the file might be mmapped.
  */
-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 */
         if (!length)
                 return 0;
+        BUG_ON((offset + length) > PAGE_CACHE_SIZE);
 
         memset(&bh, 0, sizeof(bh));
         bh.b_size = PAGE_CACHE_SIZE;
@@ -505,4 +509,26 @@ int dax_truncate_page(struct inode *inode, loff_t from, get_block_t get_block)
 
         return 0;
 }
+EXPORT_SYMBOL_GPL(dax_zero_page_range);
+
+/**
+ * dax_truncate_page - handle a partial page being truncated in a DAX file
+ * @inode: The file being truncated
+ * @from: The file offset that is being truncated to
+ * @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 a DAX file to handle the partial page.
+ *
+ * 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
+ * took care of disposing of the unnecessary blocks.  Even if the filesystem
+ * block size is smaller than PAGE_SIZE, we have to zero the rest of the page
+ * since the file might be mmapped.
+ */
+int dax_truncate_page(struct inode *inode, loff_t from, get_block_t get_block)
+{
+        unsigned length = PAGE_CACHE_ALIGN(from) - from;
+        return dax_zero_page_range(inode, from, length, get_block);
+}
 EXPORT_SYMBOL_GPL(dax_truncate_page);