* Jan Kara <jack@xxxxxxx>:
> Eric has noticed that after pagecache read rework, generic/418 is
> occasionally failing for ext4 when blocksize < pagesize. In fact, the
> pagecache rework just made hard to hit race in ext4 more likely. The
> problem is that since ext4 conversion of direct IO writes to iomap
> framework (commit 378f32bab371), we update inode size after direct IO
> write only after invalidating page cache. Thus if buffered read sneaks
> at unfortunate moment like:
>
> CPU1 - write at offset 1k CPU2 - read from offset 0
> iomap_dio_rw(..., IOMAP_DIO_FORCE_WAIT);
> ext4_readpage();
> ext4_handle_inode_extension()
>
> the read will zero out tail of the page as it still sees smaller inode
> size and thus page cache becomes inconsistent with on-disk contents with
> all the consequences.
>
> Fix the problem by moving inode size update into end_io handler which
> gets called before the page cache is invalidated.
>
> Reported-by: Eric Whitney <enwlinux@xxxxxxxxx>
> Fixes: 378f32bab371 ("ext4: introduce direct I/O write using iomap infrastructure")
> CC: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Jan Kara <jack@xxxxxxx>
> ---
> fs/ext4/file.c | 20 ++++++++++++++++----
> 1 file changed, 16 insertions(+), 4 deletions(-)
>
> Eric, can you please try whether this patch fixes the failures you are
> occasionally seeing?
Sure - the tests are running. They'll need to run overnight to get us a big
enough sample to verify the fix with a high degree of confidence. I'll report
when done.
Thanks for the patch!
Eric
>
> Changes since v1:
> * Rewritten the fix to avoid the need for separate transaction handle for
> orphan list update
>
> diff --git a/fs/ext4/file.c b/fs/ext4/file.c
> index 194f5d00fa32..be1e80af61be 100644
> --- a/fs/ext4/file.c
> +++ b/fs/ext4/file.c
> @@ -371,15 +371,27 @@ static ssize_t ext4_handle_inode_extension(struct inode *inode, loff_t offset,
> static int ext4_dio_write_end_io(struct kiocb *iocb, ssize_t size,
> int error, unsigned int flags)
> {
> - loff_t offset = iocb->ki_pos;
> + loff_t pos = iocb->ki_pos;
> struct inode *inode = file_inode(iocb->ki_filp);
>
> if (error)
> return error;
>
> - if (size && flags & IOMAP_DIO_UNWRITTEN)
> - return ext4_convert_unwritten_extents(NULL, inode,
> - offset, size);
> + if (size && flags & IOMAP_DIO_UNWRITTEN) {
> + error = ext4_convert_unwritten_extents(NULL, inode, pos, size);
> + if (error < 0)
> + return error;
> + }
> + /*
> + * If we are extending the file, we have to update i_size here before
> + * page cache gets invalidated in iomap_dio_rw(). Otherwise racing
> + * buffered reads could zero out too much from page cache pages. Update
> + * of on-disk size will happen later in ext4_dio_write_iter() where
> + * we have enough information to also perform orphan list handling etc.
> + */
> + pos += size;
> + if (pos > i_size_read(inode))
> + i_size_write(inode, pos);
>
> return 0;
> }
> --
> 2.26.2
>
