2 files changed, 57 insertions, 3 deletions

diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index 8a59f7ba30e6..bcf5d040e328 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -2287,9 +2287,22 @@ out:
 }
 
 /*
+ * Block allocation/map/preallocation routine for extents based files
+ *
+ *
  * Need to be called with
  * down_read(&EXT4_I(inode)->i_data_sem) if not allocating file system block
  * (ie, create is zero). Otherwise down_write(&EXT4_I(inode)->i_data_sem)
+ *
+ * return > 0, number of of blocks already mapped/allocated
+ *          if create == 0 and these are pre-allocated blocks
+ *              buffer head is unmapped
+ *          otherwise blocks are mapped
+ *
+ * return = 0, if plain look up failed (blocks have not been allocated)
+ *          buffer head is unmapped
+ *
+ * return < 0, error case.
  */
 int ext4_ext_get_blocks(handle_t *handle, struct inode *inode,
                         ext4_lblk_t iblock,
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 34f3eb615fd5..945cbf6cb1fc 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -908,11 +908,38 @@ out:
  */
 #define DIO_CREDITS 25
 
+
+/*
+ *
+ *
+ * ext4_ext4 get_block() wrapper function
+ * It will do a look up first, and returns if the blocks already mapped.
+ * Otherwise it takes the write lock of the i_data_sem and allocate blocks
+ * and store the allocated blocks in the result buffer head and mark it
+ * mapped.
+ *
+ * If file type is extents based, it will call ext4_ext_get_blocks(),
+ * Otherwise, call with ext4_get_blocks_handle() to handle indirect mapping
+ * based files
+ *
+ * On success, it returns the number of blocks being mapped or allocate.
+ * if create==0 and the blocks are pre-allocated and uninitialized block,
+ * the result buffer head is unmapped. If the create ==1, it will make sure
+ * the buffer head is mapped.
+ *
+ * It returns 0 if plain look up failed (blocks have not been allocated), in
+ * that casem, buffer head is unmapped
+ *
+ * It returns the error in case of allocation failure.
+ */
 int ext4_get_blocks_wrap(handle_t *handle, struct inode *inode, sector_t block,
                         unsigned long max_blocks, struct buffer_head *bh,
                         int create, int extend_disksize)
 {
         int retval;
+
+        clear_buffer_mapped(bh);
+
         /*
          * Try to see if we can get  the block without requesting
          * for new file system block.
@@ -926,12 +953,26 @@ int ext4_get_blocks_wrap(handle_t *handle, struct inode *inode, sector_t block,
                                 inode, block, max_blocks, bh, 0, 0);
         }
         up_read((&EXT4_I(inode)->i_data_sem));
-        if (!create || (retval > 0))
+
+        /* If it is only a block(s) look up */
+        if (!create)
+                return retval;
+
+        /*
+         * Returns if the blocks have already allocated
+         *
+         * Note that if blocks have been preallocated
+         * ext4_ext_get_block() returns th create = 0
+         * with buffer head unmapped.
+         */
+        if (retval > 0 && buffer_mapped(bh))
                 return retval;
 
         /*
-         * We need to allocate new blocks which will result
-         * in i_data update
+         * New blocks allocate and/or writing to uninitialized extent
+         * will possibly result in updating i_data, so we take
+         * the write lock of i_data_sem, and call get_blocks()
+         * with create == 1 flag.
          */
         down_write((&EXT4_I(inode)->i_data_sem));
         /*