linux-ext4 - Re: [RFC 1/2] iomap: direct-io: Move inode_dio_begin before filemap_write_and_wait

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Date:   Mon, 13 Jan 2020 13:51:59 -0800
From:   "Darrick J. Wong" <darrick.wong@...cle.com>
To:     Ritesh Harjani <riteshh@...ux.ibm.com>
Cc:     linux-ext4@...r.kernel.org, tytso@....edu, jack@...e.cz
Subject: Re: [RFC 1/2] iomap: direct-io: Move inode_dio_begin before
 filemap_write_and_wait_range

On Mon, Jan 13, 2020 at 04:34:21PM +0530, Ritesh Harjani wrote:
> Some filesystems (e.g. ext4) need to know in it's writeback path, that
> whether DIO is in progress or not. This info may be needed to avoid the
> stale data exposure race with DIO reads.

Does XFS have this problem too?

Admittedly dio read during mmap write is probably not well supported. ;)

> Signed-off-by: Ritesh Harjani <riteshh@...ux.ibm.com>
> ---
>  fs/iomap/direct-io.c | 17 +++++++++++++----

Might want to cc fsdevel and the iomap maintainers...

--D

>  1 file changed, 13 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
> index 23837926c0c5..d1c159bd3854 100644
> --- a/fs/iomap/direct-io.c
> +++ b/fs/iomap/direct-io.c
> @@ -468,9 +468,18 @@ iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
>  		flags |= IOMAP_NOWAIT;
>  	}
>  
> +	/*
> +	 * Call inode_dio_begin() before we write out and wait for writeback to
> +	 * complete. This may be needed by some filesystems to prevent race
> +	 * like stale data exposure by DIO reads.
> +	 */
> +	inode_dio_begin(inode);
> +	/* So that i_dio_count is incremented before below operation */
> +	smp_mb__after_atomic();
> +
>  	ret = filemap_write_and_wait_range(mapping, pos, end);
>  	if (ret)
> -		goto out_free_dio;
> +		goto out_end_dio;
>  
>  	/*
>  	 * Try to invalidate cache pages for the range we're direct
> @@ -488,11 +497,9 @@ iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
>  	    !inode->i_sb->s_dio_done_wq) {
>  		ret = sb_init_dio_done_wq(inode->i_sb);
>  		if (ret < 0)
> -			goto out_free_dio;
> +			goto out_end_dio;
>  	}
>  
> -	inode_dio_begin(inode);
> -
>  	blk_start_plug(&plug);
>  	do {
>  		ret = iomap_apply(inode, pos, count, flags, ops, dio,
> @@ -568,6 +575,8 @@ iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
>  
>  	return iomap_dio_complete(dio);
>  
> +out_end_dio:
> +	inode_dio_end(inode);
>  out_free_dio:
>  	kfree(dio);
>  	return ret;
> -- 
> 2.21.0
>