Re: [PATCH v5 4/9] fs: have setattr_copy handle multigrain timestamps appropriately

["Darrick J. Wong" <djwong@kernel.org>]

On Thu, Jul 11, 2024 at 07:08:08AM -0400, Jeff Layton wrote:
> The setattr codepath is still using coarse-grained timestamps, even on
> multigrain filesystems. To fix this, we need to fetch the timestamp for
> ctime updates later, at the point where the assignment occurs in
> setattr_copy.
> 
> On a multigrain inode, ignore the ia_ctime in the attrs, and always
> update the ctime to the current clock value. Update the atime and mtime
> with the same value (if needed) unless they are being set to other
> specific values, a'la utimes().
> 
> Note that we don't want to do this universally however, as some
> filesystems (e.g. most networked fs) want to do an explicit update
> elsewhere before updating the local inode.
> 
> Signed-off-by: Jeff Layton <jlayton@kernel.org>

Makes sense to me,
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

--D

> ---
>  fs/attr.c | 52 ++++++++++++++++++++++++++++++++++++++++++++++------
>  1 file changed, 46 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/attr.c b/fs/attr.c
> index 825007d5cda4..e03ea6951864 100644
> --- a/fs/attr.c
> +++ b/fs/attr.c
> @@ -271,6 +271,42 @@ int inode_newsize_ok(const struct inode *inode, loff_t offset)
>  }
>  EXPORT_SYMBOL(inode_newsize_ok);
>  
> +/**
> + * setattr_copy_mgtime - update timestamps for mgtime inodes
> + * @inode: inode timestamps to be updated
> + * @attr: attrs for the update
> + *
> + * With multigrain timestamps, we need to take more care to prevent races
> + * when updating the ctime. Always update the ctime to the very latest
> + * using the standard mechanism, and use that to populate the atime and
> + * mtime appropriately (unless we're setting those to specific values).
> + */
> +static void setattr_copy_mgtime(struct inode *inode, const struct iattr *attr)
> +{
> +	unsigned int ia_valid = attr->ia_valid;
> +	struct timespec64 now;
> +
> +	/*
> +	 * If the ctime isn't being updated then nothing else should be
> +	 * either.
> +	 */
> +	if (!(ia_valid & ATTR_CTIME)) {
> +		WARN_ON_ONCE(ia_valid & (ATTR_ATIME|ATTR_MTIME));
> +		return;
> +	}
> +
> +	now = inode_set_ctime_current(inode);
> +	if (ia_valid & ATTR_ATIME_SET)
> +		inode_set_atime_to_ts(inode, attr->ia_atime);
> +	else if (ia_valid & ATTR_ATIME)
> +		inode_set_atime_to_ts(inode, now);
> +
> +	if (ia_valid & ATTR_MTIME_SET)
> +		inode_set_mtime_to_ts(inode, attr->ia_mtime);
> +	else if (ia_valid & ATTR_MTIME)
> +		inode_set_mtime_to_ts(inode, now);
> +}
> +
>  /**
>   * setattr_copy - copy simple metadata updates into the generic inode
>   * @idmap:	idmap of the mount the inode was found from
> @@ -303,12 +339,6 @@ void setattr_copy(struct mnt_idmap *idmap, struct inode *inode,
>  
>  	i_uid_update(idmap, attr, inode);
>  	i_gid_update(idmap, attr, inode);
> -	if (ia_valid & ATTR_ATIME)
> -		inode_set_atime_to_ts(inode, attr->ia_atime);
> -	if (ia_valid & ATTR_MTIME)
> -		inode_set_mtime_to_ts(inode, attr->ia_mtime);
> -	if (ia_valid & ATTR_CTIME)
> -		inode_set_ctime_to_ts(inode, attr->ia_ctime);
>  	if (ia_valid & ATTR_MODE) {
>  		umode_t mode = attr->ia_mode;
>  		if (!in_group_or_capable(idmap, inode,
> @@ -316,6 +346,16 @@ void setattr_copy(struct mnt_idmap *idmap, struct inode *inode,
>  			mode &= ~S_ISGID;
>  		inode->i_mode = mode;
>  	}
> +
> +	if (is_mgtime(inode))
> +		return setattr_copy_mgtime(inode, attr);
> +
> +	if (ia_valid & ATTR_ATIME)
> +		inode_set_atime_to_ts(inode, attr->ia_atime);
> +	if (ia_valid & ATTR_MTIME)
> +		inode_set_mtime_to_ts(inode, attr->ia_mtime);
> +	if (ia_valid & ATTR_CTIME)
> +		inode_set_ctime_to_ts(inode, attr->ia_ctime);
>  }
>  EXPORT_SYMBOL(setattr_copy);
>  
> 
> -- 
> 2.45.2
> 
>