Re: [PATCH v2] ext4: Fix occasional generic/418 failure

[Dave Chinner <david@fromorbit.com>]

On Wed, Apr 14, 2021 at 03:14:53PM +0200, Jan Kara wrote:
> 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@gmail.com>
> Fixes: 378f32bab371 ("ext4: introduce direct I/O write using iomap infrastructure")
> CC: stable@vger.kernel.org
> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
>  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?
> 
> 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);

Might be worth explaining why this doesn't require locking to
prevent racing completions from updating the inode size and
potentially losing an EOF update. I know why but it might not be so
obvious to others (DIO extending writes are serialised
at submission in ext4) but it's probably worth having a comment
similar to the one in xfs_dio_write_end_io() that explains why XFS
needs locking.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com