2 files changed, 33 insertions, 2 deletions

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index 2f310d1839ed..c48f9db5b96b 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -1825,6 +1825,8 @@ extern int ext4_writepage_trans_blocks(struct inode *);
 extern int ext4_chunk_trans_blocks(struct inode *, int nrblocks);
 extern int ext4_block_truncate_page(handle_t *handle,
                 struct address_space *mapping, loff_t from);
+extern int ext4_block_zero_page_range(handle_t *handle,
+                struct address_space *mapping, loff_t from, loff_t length);
 extern int ext4_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf);
 extern qsize_t *ext4_get_reserved_space(struct inode *inode);
 extern void ext4_da_update_reserve_space(struct inode *inode,
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index aef864b725d3..fbe11e676624 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -3914,9 +3914,30 @@ void ext4_set_aops(struct inode *inode)
 int ext4_block_truncate_page(handle_t *handle,
                 struct address_space *mapping, loff_t from)
 {
+        unsigned offset = from & (PAGE_CACHE_SIZE-1);
+        unsigned length;
+        unsigned blocksize;
+        struct inode *inode = mapping->host;
+
+        blocksize = inode->i_sb->s_blocksize;
+        length = blocksize - (offset & (blocksize - 1));
+
+        return ext4_block_zero_page_range(handle, mapping, from, length);
+}
+
+/*
+ * ext4_block_zero_page_range() zeros out a mapping of length 'length'
+ * starting from file offset 'from'.  The range to be zero'd must
+ * be contained with in one block.  If the specified range exceeds
+ * the end of the block it will be shortened to end of the block
+ * that cooresponds to 'from'
+ */
+int ext4_block_zero_page_range(handle_t *handle,
+                struct address_space *mapping, loff_t from, loff_t length)
+{
         ext4_fsblk_t index = from >> PAGE_CACHE_SHIFT;
         unsigned offset = from & (PAGE_CACHE_SIZE-1);
-        unsigned blocksize, length, pos;
+        unsigned blocksize, max, pos;
         ext4_lblk_t iblock;
         struct inode *inode = mapping->host;
         struct buffer_head *bh;
@@ -3929,7 +3950,15 @@ int ext4_block_truncate_page(handle_t *handle,
                 return -EINVAL;
 
         blocksize = inode->i_sb->s_blocksize;
-        length = blocksize - (offset & (blocksize - 1));
+        max = blocksize - (offset & (blocksize - 1));
+
+        /*
+         * correct length if it does not fall between
+         * 'from' and the end of the block
+         */
+        if (length > max || length < 0)
+                length = max;
+
         iblock = index << (PAGE_CACHE_SHIFT - inode->i_sb->s_blocksize_bits);
 
         if (!page_has_buffers(page))