2 files changed, 49 insertions, 0 deletions

diff --git a/fs/buffer.c b/fs/buffer.c
index 0f9006714230..02ebb1f1d3b0 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -2194,6 +2194,52 @@ int generic_commit_write(struct file *file, struct page *page,
         return 0;
 }
 
+/*
+ * block_page_mkwrite() is not allowed to change the file size as it gets
+ * called from a page fault handler when a page is first dirtied. Hence we must
+ * be careful to check for EOF conditions here. We set the page up correctly
+ * for a written page which means we get ENOSPC checking when writing into
+ * holes and correct delalloc and unwritten extent mapping on filesystems that
+ * support these features.
+ *
+ * We are not allowed to take the i_mutex here so we have to play games to
+ * protect against truncate races as the page could now be beyond EOF.  Because
+ * vmtruncate() writes the inode size before removing pages, once we have the
+ * page lock we can determine safely if the page is beyond EOF. If it is not
+ * beyond EOF, then the page is guaranteed safe against truncation until we
+ * unlock the page.
+ */
+int
+block_page_mkwrite(struct vm_area_struct *vma, struct page *page,
+                   get_block_t get_block)
+{
+        struct inode *inode = vma->vm_file->f_path.dentry->d_inode;
+        unsigned long end;
+        loff_t size;
+        int ret = -EINVAL;
+
+        lock_page(page);
+        size = i_size_read(inode);
+        if ((page->mapping != inode->i_mapping) ||
+            ((page->index << PAGE_CACHE_SHIFT) > size)) {
+                /* page got truncated out from underneath us */
+                goto out_unlock;
+        }
+
+        /* page is wholly or partially inside EOF */
+        if (((page->index + 1) << PAGE_CACHE_SHIFT) > size)
+                end = size & ~PAGE_CACHE_MASK;
+        else
+                end = PAGE_CACHE_SIZE;
+
+        ret = block_prepare_write(page, 0, end, get_block);
+        if (!ret)
+                ret = block_commit_write(page, 0, end);
+
+out_unlock:
+        unlock_page(page);
+        return ret;
+}
 
 /*
  * nobh_prepare_write()'s prereads are special: the buffer_heads are freed
@@ -2977,6 +3023,7 @@ EXPORT_SYMBOL(__brelse);
 EXPORT_SYMBOL(__wait_on_buffer);
 EXPORT_SYMBOL(block_commit_write);
 EXPORT_SYMBOL(block_prepare_write);
+EXPORT_SYMBOL(block_page_mkwrite);
 EXPORT_SYMBOL(block_read_full_page);
 EXPORT_SYMBOL(block_sync_page);
 EXPORT_SYMBOL(block_truncate_page);
diff --git a/include/linux/buffer_head.h b/include/linux/buffer_head.h
index 5c6e12853a9b..35cadad84b14 100644
--- a/include/linux/buffer_head.h
+++ b/include/linux/buffer_head.h
@@ -209,6 +209,8 @@ int cont_prepare_write(struct page*, unsigned, unsigned, get_block_t*,
 int generic_cont_expand(struct inode *inode, loff_t size);
 int generic_cont_expand_simple(struct inode *inode, loff_t size);
 int block_commit_write(struct page *page, unsigned from, unsigned to);
+int block_page_mkwrite(struct vm_area_struct *vma, struct page *page,
+                                get_block_t get_block);
 void block_sync_page(struct page *);
 sector_t generic_block_bmap(struct address_space *, sector_t, get_block_t *);
 int generic_commit_write(struct file *, struct page *, unsigned, unsigned);