ext4: make ext4_ext_get_blocks always return <= max_blocks

ext4_ext_get_blocks() returns number of blocks allocated with buffer
heads 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, since some of the code paths cannot
handle such a return value.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

1 files changed, 11 insertions, 1 deletions

diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index c2b004e29ad8..f4ef0b745a53 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -2570,8 +2570,18 @@ 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 returns 0s.
+                                 */
+                                if (allocated > max_blocks)
+                                        allocated = max_blocks;
                                 goto out2;
+                        }
 
                         ret = ext4_ext_convert_to_initialized(handle, inode,
                                                                 path, iblock,