Re: [dm-devel] [PATCH v13 4/9] fs, block: copy_file_range for def_blk_ops for direct block device

From: Christoph Hellwig <hch@lst.de>
To: Nitesh Shetty <nj.shetty@samsung.com>
Cc: linux-doc@vger.kernel.org, djwong@kernel.org,
	linux-nvme@lists.infradead.org, dm-devel@redhat.com,
	Christoph Hellwig <hch@lst.de>, Alasdair Kergon <agk@redhat.com>,
	Sagi Grimberg <sagi@grimberg.me>,
	linux-scsi@vger.kernel.org, Jonathan Corbet <corbet@lwn.net>,
	gost.dev@samsung.com, nitheshshetty@gmail.com,
	willy@infradead.org, Chaitanya Kulkarni <kch@nvidia.com>,
	Anuj Gupta <anuj20.g@samsung.com>,
	Mike Snitzer <snitzer@kernel.org>,
	ming.lei@redhat.com, linux-block@vger.kernel.org,
	dlemoal@kernel.org, Alexander Viro <viro@zeniv.linux.org.uk>,
	Keith Busch <kbusch@kernel.org>,
	bvanassche@acm.org, Jens Axboe <axboe@kernel.dk>,
	Christian Brauner <brauner@kernel.org>,
	martin.petersen@oracle.com, linux-kernel@vger.kernel.org,
	linux-fsdevel@vger.kernel.org
Subject: Re: [dm-devel] [PATCH v13 4/9] fs, block: copy_file_range for def_blk_ops for direct block device
Date: Thu, 20 Jul 2023 09:57:10 +0200	[thread overview]
Message-ID: <20230720075710.GC5042@lst.de> (raw)
In-Reply-To: <20230627183629.26571-5-nj.shetty@samsung.com>

> +/* Copy source offset from source block device to destination block
> + * device. Returns the length of bytes copied.
> + */
> +ssize_t blkdev_copy_offload_failfast(
> +		struct block_device *bdev_in, loff_t pos_in,
> +		struct block_device *bdev_out, loff_t pos_out,
> +		size_t len, gfp_t gfp_mask)

This is an odd and very misnamed interface.

Either we have a klkdev_copy() interface that automatically falls back
to a fallback (maybe with an opt-out), or we have separate
blkdev_copy_offload/blkdev_copy_emulated interface and let the caller
decide.  But none of that really is "failfast".

Also this needs to go into the helpers patch and not a patch that is
supposed to just wire copying up for block device node.

> index b07de77ef126..d27148a2543f 100644
> --- a/fs/read_write.c
> +++ b/fs/read_write.c
> @@ -1447,7 +1447,8 @@ static int generic_copy_file_checks(struct file *file_in, loff_t pos_in,
>  		return -EOVERFLOW;
>  
>  	/* Shorten the copy to EOF */
> -	size_in = i_size_read(inode_in);
> +	size_in = i_size_read(file_in->f_mapping->host);

generic_copy_file_checks needs to be fixed to use ->mapping->host both
or inode_in and inode_out at the top of the file instead of this
band aid.  And that needs to be a separate patch with a Fixes tag.

> @@ -1708,7 +1709,9 @@ int generic_file_rw_checks(struct file *file_in, struct file *file_out)
>  	/* Don't copy dirs, pipes, sockets... */
>  	if (S_ISDIR(inode_in->i_mode) || S_ISDIR(inode_out->i_mode))
>  		return -EISDIR;
> -	if (!S_ISREG(inode_in->i_mode) || !S_ISREG(inode_out->i_mode))
> +
> +	if ((!S_ISREG(inode_in->i_mode) || !S_ISREG(inode_out->i_mode)) &&
> +		(!S_ISBLK(inode_in->i_mode) || !S_ISBLK(inode_out->i_mode)))

This is using weird indentation, and might also not be doing
exactly what we want.  I think the better thing to do here is to:

 1) check for the accetable types only on the in inode
 2) have a check that the mode matches for the in and out inodes

And please do this as a separate prep patch instead of hiding it here.

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