ext4: Fix sub-block zeroing for writes into preallocated extents

We need to mark the buffer_head mapping preallocated space as new
during write_begin. Otherwise we don't zero out the page cache content
properly for a partial write. This will cause file corruption with
preallocation.

Now that we mark the buffer_head new we also need to have a valid
buffer_head blocknr so that unmap_underlying_metadata() unmaps the
correct block.

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

2 files changed, 9 insertions, 0 deletions

diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index e40332158340..172656c2a3bd 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -2875,6 +2875,8 @@ int ext4_ext_get_blocks(handle_t *handle, struct inode *inode,
                                 if (allocated > max_blocks)
                                         allocated = max_blocks;
                                 set_buffer_unwritten(bh_result);
+                                bh_result->b_bdev = inode->i_sb->s_bdev;
+                                bh_result->b_blocknr = newblock;
                                 goto out2;
                         }
 
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index e91f978c7f12..d4b634ae06bd 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -2323,6 +2323,13 @@ static int ext4_da_get_block_prep(struct inode *inode, sector_t iblock,
                 set_buffer_delay(bh_result);
         } else if (ret > 0) {
                 bh_result->b_size = (ret << inode->i_blkbits);
+                /*
+                 * With sub-block writes into unwritten extents
+                 * we also need to mark the buffer as new so that
+                 * the unwritten parts of the buffer gets correctly zeroed.
+                 */
+                if (buffer_unwritten(bh_result))
+                        set_buffer_new(bh_result);
                 ret = 0;
         }