linux-ext4 - Re: [PATCH 6/8] nowait aio: ext4

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Date:   Tue, 4 Apr 2017 09:58:53 +0200
From:   Jan Kara <jack@...e.cz>
To:     Goldwyn Rodrigues <rgoldwyn@...e.de>
Cc:     linux-fsdevel@...r.kernel.org, jack@...e.com, hch@...radead.org,
        linux-block@...r.kernel.org, linux-btrfs@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org,
        sagi@...mberg.me, avi@...lladb.com, axboe@...nel.dk,
        linux-api@...r.kernel.org, willy@...radead.org,
        tom.leiming@...il.com, Goldwyn Rodrigues <rgoldwyn@...e.com>
Subject: Re: [PATCH 6/8] nowait aio: ext4

On Mon 03-04-17 13:53:05, Goldwyn Rodrigues wrote:
> From: Goldwyn Rodrigues <rgoldwyn@...e.com>
> 
> Return EAGAIN if any of the following checks fail for direct I/O:
>  + i_rwsem is lockable
>  + Writing beyond end of file (will trigger allocation)
>  + Blocks are not allocated at the write location

Patches seem to be missing your Signed-off-by tag...

> @@ -235,9 +237,21 @@ ext4_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
>  
>  	iocb->private = &overwrite;
>  	/* Check whether we do a DIO overwrite or not */
> -	if (o_direct && ext4_should_dioread_nolock(inode) && !unaligned_aio &&
> -	    ext4_overwrite_io(inode, iocb->ki_pos, iov_iter_count(from)))
> -		overwrite = 1;
> +	if (o_direct && !unaligned_aio) {
> +		struct ext4_map_blocks map;
> +		if (ext4_blocks_mapped(inode, iocb->ki_pos,
> +				      iov_iter_count(from), &map)) {
> +	 		/* To exclude unwritten extents, we need to check
> +			 * m_flags.
> +			 */
> +			if (ext4_should_dioread_nolock(inode) &&
> +			    (map.m_flags & EXT4_MAP_MAPPED))
> +				overwrite = 1;
> +		} else if (iocb->ki_flags & IOCB_NOWAIT) {
> +			ret = -EAGAIN;
> +			goto out;
> +		}
> +	}

Actually, overwriting unwritten extents is relatively complex in ext4 as
well. In particular we need to start a transaction and split out the
written part of the extent. So I don't think we can easily support this
without blocking. As a result I'd keep the condition for IOCB_NOWAIT the
same as for overwrite IO.

> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -117,7 +117,7 @@ static struct file_system_type ext2_fs_type = {
>  	.name		= "ext2",
>  	.mount		= ext4_mount,
>  	.kill_sb	= kill_block_super,
> -	.fs_flags	= FS_REQUIRES_DEV,
> +	.fs_flags	= FS_REQUIRES_DEV | FS_NOWAIT,

FS_NOWAIT looks a bit too generic given these are filesystem feature flags.
Can we call it FS_NOWAIT_IO?

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