[RFC PATCH] ext4: Return unwritten buffer head when trying to read from prealloc space.

["Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>]

ext4_ext_get_blocks  returns number of blocks allocated with buffer head
unmapped for a read from prealloc space. This is needed so that delayed
allocation doesn't do block reservation for prealloc space since the blocks
are already resevred on disk. Fix ext4_ext_get_blocks to not return greater
than max_blocks and also mark the buffer head unwritten. Some code path tries
to read the block if buffer_head is not new and no uptodate. Marking the buffer
head unwritten avoid this reading.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
---
 fs/ext4/extents.c |   14 +++++++++++++-
 1 files changed, 13 insertions(+), 1 deletions(-)

diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index d6ae40a..edd1bf2 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -2600,8 +2600,20 @@ int ext4_ext_get_blocks(handle_t *handle, struct inode *inode,
 			}
 			if (create == EXT4_CREATE_UNINITIALIZED_EXT)
 				goto out;
-			if (!create)
+			if (!create) {
+				/*
+				 * We have blocks reserved already. We
+				 * return allocated blocks so that delalloc
+				 * won't do block reservation for us. But
+				 * the buffer head will be unmapped so that
+				 * a read from the block return 0
+				 */
+				if (allocated > max_blocks)
+					allocated = max_blocks;
+				/* mark the buffer unwritten */
+				__set_bit(BH_Unwritten, &bh_result->b_state);
 				goto out2;
+			}
 
 			ret = ext4_ext_convert_to_initialized(handle, inode,
 								path, iblock,
-- 
1.5.5.rc0.16.g02b00.dirty