1 files changed, 46 insertions, 0 deletions

diff --git a/fs/buffer.c b/fs/buffer.c
index f95805019639..ca12a6bb82b1 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -2096,6 +2096,52 @@ int generic_write_end(struct file *file, struct address_space *mapping,
 EXPORT_SYMBOL(generic_write_end);
 
 /*
+ * block_is_partially_uptodate checks whether buffers within a page are
+ * uptodate or not.
+ *
+ * Returns true if all buffers which correspond to a file portion
+ * we want to read are uptodate.
+ */
+int block_is_partially_uptodate(struct page *page, read_descriptor_t *desc,
+                                        unsigned long from)
+{
+        struct inode *inode = page->mapping->host;
+        unsigned block_start, block_end, blocksize;
+        unsigned to;
+        struct buffer_head *bh, *head;
+        int ret = 1;
+
+        if (!page_has_buffers(page))
+                return 0;
+
+        blocksize = 1 << inode->i_blkbits;
+        to = min_t(unsigned, PAGE_CACHE_SIZE - from, desc->count);
+        to = from + to;
+        if (from < blocksize && to > PAGE_CACHE_SIZE - blocksize)
+                return 0;
+
+        head = page_buffers(page);
+        bh = head;
+        block_start = 0;
+        do {
+                block_end = block_start + blocksize;
+                if (block_end > from && block_start < to) {
+                        if (!buffer_uptodate(bh)) {
+                                ret = 0;
+                                break;
+                        }
+                        if (block_end >= to)
+                                break;
+                }
+                block_start = block_end;
+                bh = bh->b_this_page;
+        } while (bh != head);
+
+        return ret;
+}
+EXPORT_SYMBOL(block_is_partially_uptodate);
+
+/*
  * Generic "read page" function for block devices that have the normal
  * get_block functionality. This is most of the block device filesystems.
  * Reads the page asynchronously --- the unlock_buffer() and