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

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Date:   Tue, 14 Jan 2020 10:12:08 +0100
From:   Jan Kara <jack@...e.cz>
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 13-01-20 16:34:21, 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.
> 
> Signed-off-by: Ritesh Harjani <riteshh@...ux.ibm.com>
> ---
>  fs/iomap/direct-io.c | 17 +++++++++++++----
>  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();

I wonder if the barrier shouldn't go into inode_dio_begin() - as a sepatare
patch. Because people just treat this as a lock-kind-of-thingy. E.g. btrfs
or ceph use inode_dio_begin() in places which I'd consider prone to CPU
reordering issues without this barrier...

Otherwise the patch looks good to me.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR