Re: [PATCH 08/10] fs: add support for non-blocking timestamp updates

[Jan Kara <jack@suse.cz>]

On Wed 17-12-25 07:09:41, Christoph Hellwig wrote:
> Currently file_update_time_flags unconditionally returns -EAGAIN if any
> timestamp needs to be updated and IOCB_NOWAIT is passed.  This makes
> non-blocking direct writes impossible on file systems with granular
> enough timestamps.
> 
> Add a S_NOWAIT to ask for timestamps to not block, and return -EAGAIN in
> all methods for now.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> Reviewed-by: Jeff Layton <jlayton@kernel.org>

...

> @@ -2110,12 +2110,26 @@ int inode_update_timestamps(struct inode *inode, int *flags)
>  		now = inode_set_ctime_current(inode);
>  		if (!timespec64_equal(&now, &ctime))
>  			updated |= S_CTIME;
> -		if (!timespec64_equal(&now, &mtime)) {
> -			inode_set_mtime_to_ts(inode, now);
> +		if (!timespec64_equal(&now, &mtime))
>  			updated |= S_MTIME;
> +
> +		if (IS_I_VERSION(inode)) {
> +			if (*flags & S_NOWAIT) {
> +				/*
> +				 * Error out if we'd need timestamp updates, as
> +				 * the generally requires blocking to dirty the
> +				 * inode in one form or another.
> +				 */
> +				if (updated && inode_iversion_need_inc(inode))
> +					goto bail;

I'm confused here. What the code does is that if S_NOWAIT is set and
i_version needs increment we bail. However the comment as well as the
changelog speaks about timestamps needing update and not about i_version.
And intuitively I agree that if any timestamp is updated, inode needs
dirtying and thus we should bail regardless of whether i_version is updated
as well or not. What am I missing?

								Honza

> +			} else {
> +				if (inode_maybe_inc_iversion(inode, updated))
> +					updated |= S_VERSION;
> +			}
>  		}
> -		if (IS_I_VERSION(inode) && inode_maybe_inc_iversion(inode, updated))
> -			updated |= S_VERSION;
> +
> +		if (updated & S_MTIME)
> +			inode_set_mtime_to_ts(inode, now);
>  	} else {
>  		now = current_time(inode);
>  	}
> @@ -2131,6 +2145,9 @@ int inode_update_timestamps(struct inode *inode, int *flags)
>  
>  	*flags = updated;
>  	return 0;
> +bail:
> +	*flags = 0;
> +	return -EAGAIN;
>  }
>  EXPORT_SYMBOL(inode_update_timestamps);
>  
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR