Re: [patch 6/7] fs: fsync optimisations

[Christoph Hellwig]

On Wed, Nov 24, 2010 at 01:06:16AM +1100, npiggin@xxxxxxxxx wrote:
> Optimise fsync by adding a datasync parameter to sync_inode_metadata to
> DTRT with writing back the inode (->write_inode in theory should have a
> datasync parameter too perhaps, but that's for another time).
> 
> Also, implement the metadata sync optimally rather than reusing the
> normal data writeback path. This means less useless moving the inode around the
> writeback lists, and less dropping and retaking of inode_lock, and avoiding
> the data writeback call with nr_pages == 0.

This patch looks good to me, but a few minor comments below:

> @@ -1315,13 +1315,49 @@ EXPORT_SYMBOL(sync_inode);
>   *
>   * Note: only writes the actual inode, no associated data or other metadata.
>   */
> -int sync_inode_metadata(struct inode *inode, int wait)
> +int sync_inode_metadata(struct inode *inode, int datasync, int wait)
>  {
> +	struct address_space *mapping = inode->i_mapping;
>  	struct writeback_control wbc = {
>  		.sync_mode = wait ? WB_SYNC_ALL : WB_SYNC_NONE,
>  		.nr_to_write = 0, /* metadata-only */
>  	};
> +	unsigned dirty, mask;
> +	int ret = 0;
>  
> -	return sync_inode(inode, &wbc);
> +	/*
> +	 * This is a similar implementation to writeback_single_inode.
> +	 * Keep them in sync.
> +	 */

I'd move this comment to above the function.

> +	/*
> +	 * Generic write_inode doesn't distinguish between sync and datasync,
> +	 * so even a datasync can clear the sync state. Filesystems which
> +	 * distiguish these cases must only clear 'mask' in their metadata
> +	 * sync code.
> +	 */

I don't understand this comment.  Currenly filesystems never clear
i_state bits by themselves.

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