The syzbot has reported that it can hit the warning in
ext4_dio_write_end_io() because i_size < i_disksize. Indeed the
reproducer creates a race between DIO IO completion and truncate
expanding the file and thus ext4_dio_write_end_io() sees an inconsistent
inode state where i_disksize is already updated but i_size is not
updated yet. Since we are careful when setting up DIO write and consider
it extending (and thus performing the IO synchronously with i_rwsem held
exclusively) whenever it goes past either of i_size or i_disksize, we
can use the same test during IO completion without risking entering
ext4_handle_inode_extension() without i_rwsem held. This way we make it
obvious both i_size and i_disksize are large enough when we report DIO
completion without relying on unreliable WARN_ON.
Reported-by: syzbot+47479b71cdfc78f56d30@xxxxxxxxxxxxxxxxxxxxxxxxx
Fixes: 91562895f803 ("ext4: properly sync file size update after O_SYNC direct IO")
Signed-off-by: Jan Kara <jack@xxxxxxx>
---
fs/ext4/file.c | 7 ++++---
1 file changed, 4 insertions(+), 3 deletions(-)
diff --git a/fs/ext4/file.c b/fs/ext4/file.c
index 0166bb9ca160..ba497aabdd1e 100644
--- a/fs/ext4/file.c
+++ b/fs/ext4/file.c
@@ -386,10 +386,11 @@ static int ext4_dio_write_end_io(struct kiocb *iocb, ssize_t size,
* blocks. But the code in ext4_iomap_alloc() is careful to use
* zeroed/unwritten extents if this is possible; thus we won't leave
* uninitialized blocks in a file even if we didn't succeed in writing
- * as much as we intended.
+ * as much as we intended. Also we can race with truncate or write
+ * expanding the file so we have to be a bit careful here.
*/
- WARN_ON_ONCE(i_size_read(inode) < READ_ONCE(EXT4_I(inode)->i_disksize));
- if (pos + size <= READ_ONCE(EXT4_I(inode)->i_disksize))
+ if (pos + size <= READ_ONCE(EXT4_I(inode)->i_disksize) &&
+ pos + size <= i_size_read(inode))
return size;
return ext4_handle_inode_extension(inode, pos, size);
}
--
2.35.3
