Re: [PATCH v2 1/3] fs: add infrastructure for multigrain inode i_m/ctime

[Christian Brauner <brauner@kernel.org>]

On Mon, Apr 24, 2023 at 11:11:02AM -0400, Jeff Layton wrote:
> The VFS always uses coarse-grained timestamp updates for filling out the
> ctime and mtime after a change. This has the benefit of allowing
> filesystems to optimize away a lot metaupdates, to around once per
> jiffy, even when a file is under heavy writes.
> 
> Unfortunately, this has always been an issue when we're exporting via
> NFSv3, which relies on timestamps to validate caches. Even with NFSv4, a
> lot of exported filesystems don't properly support a change attribute
> and are subject to the same problems with timestamp granularity. Other
> applications have similar issues (e.g backup applications).
> 
> Switching to always using fine-grained timestamps would improve the
> situation for NFS, but that becomes rather expensive, as the underlying
> filesystem will have to log a lot more metadata updates.
> 
> What we need is a way to only use fine-grained timestamps when they are
> being actively queried:
> 
> Whenever the mtime changes, the ctime must also change since we're
> changing the metadata. When a superblock has a s_time_gran >1, we can
> use the lowest-order bit of the inode->i_ctime as a flag to indicate
> that the value has been queried. Then on the next write, we'll fetch a
> fine-grained timestamp instead of the usual coarse-grained one.
> 
> We could enable this for any filesystem that has a s_time_gran >1, but
> for now, this patch adds a new SB_MULTIGRAIN_TS flag to allow filesystems
> to opt-in to this behavior.
> 
> It then adds a new current_ctime function that acts like the
> current_time helper, but will conditionally grab fine-grained timestamps
> when the flag is set in the current ctime. Also, there is a new
> generic_fill_multigrain_cmtime for grabbing the c/mtime out of the inode
> and atomically marking the ctime as queried.
> 
> Later patches will convert filesystems over to this new scheme.
> 
> Signed-off-by: Jeff Layton <jlayton@kernel.org>
> ---
>  fs/inode.c         | 57 +++++++++++++++++++++++++++++++++++++++---
>  fs/stat.c          | 24 ++++++++++++++++++
>  include/linux/fs.h | 62 ++++++++++++++++++++++++++++++++--------------
>  3 files changed, 121 insertions(+), 22 deletions(-)
> 
> diff --git a/fs/inode.c b/fs/inode.c
> index 4558dc2f1355..4bd11bdb46d4 100644
> --- a/fs/inode.c
> +++ b/fs/inode.c
> @@ -2030,6 +2030,7 @@ EXPORT_SYMBOL(file_remove_privs);
>  static int inode_needs_update_time(struct inode *inode, struct timespec64 *now)
>  {
>  	int sync_it = 0;
> +	struct timespec64 ctime = inode->i_ctime;
>  
>  	/* First try to exhaust all avenues to not sync */
>  	if (IS_NOCMTIME(inode))
> @@ -2038,7 +2039,9 @@ static int inode_needs_update_time(struct inode *inode, struct timespec64 *now)
>  	if (!timespec64_equal(&inode->i_mtime, now))
>  		sync_it = S_MTIME;
>  
> -	if (!timespec64_equal(&inode->i_ctime, now))
> +	if (is_multigrain_ts(inode))
> +		ctime.tv_nsec &= ~I_CTIME_QUERIED;
> +	if (!timespec64_equal(&ctime, now))
>  		sync_it |= S_CTIME;
>  
>  	if (IS_I_VERSION(inode) && inode_iversion_need_inc(inode))
> @@ -2062,6 +2065,50 @@ static int __file_update_time(struct file *file, struct timespec64 *now,
>  	return ret;
>  }
>  
> +/**
> + * current_ctime - Return FS time (possibly high-res)
> + * @inode: inode.
> + *
> + * Return the current time truncated to the time granularity supported by
> + * the fs, as suitable for a ctime/mtime change.
> + *
> + * For a multigrain timestamp, if the timestamp is flagged as having been
> + * QUERIED, then get a fine-grained timestamp.
> + */
> +struct timespec64 current_ctime(struct inode *inode)
> +{
> +	struct timespec64 now;
> +	long nsec = 0;
> +	bool multigrain = is_multigrain_ts(inode);
> +
> +	if (multigrain) {
> +		atomic_long_t *pnsec = (atomic_long_t *)&inode->i_ctime.tv_nsec;
> +
> +		nsec = atomic_long_fetch_and(~I_CTIME_QUERIED, pnsec);
> +	}
> +
> +	if (nsec & I_CTIME_QUERIED) {
> +		ktime_get_real_ts64(&now);
> +	} else {
> +		ktime_get_coarse_real_ts64(&now);
> +
> +		if (multigrain) {
> +			/*
> +			 * If we've recently fetched a fine-grained timestamp
> +			 * then the coarse-grained one may be earlier than the
> +			 * existing one. Just keep the existing ctime if so.
> +			 */
> +			struct timespec64 ctime = inode->i_ctime;
> +
> +			if (timespec64_compare(&ctime, &now) > 0)
> +				now = ctime;
> +		}
> +	}
> +
> +	return timestamp_truncate(now, inode);
> +}
> +EXPORT_SYMBOL(current_ctime);
> +
>  /**
>   * file_update_time - update mtime and ctime time
>   * @file: file accessed
> @@ -2080,7 +2127,7 @@ int file_update_time(struct file *file)
>  {
>  	int ret;
>  	struct inode *inode = file_inode(file);
> -	struct timespec64 now = current_time(inode);
> +	struct timespec64 now = current_ctime(inode);
>  
>  	ret = inode_needs_update_time(inode, &now);
>  	if (ret <= 0)
> @@ -2109,7 +2156,7 @@ static int file_modified_flags(struct file *file, int flags)
>  {
>  	int ret;
>  	struct inode *inode = file_inode(file);
> -	struct timespec64 now = current_time(inode);
> +	struct timespec64 now = current_ctime(inode);
>  
>  	/*
>  	 * Clear the security bits if the process is not being run by root.
> @@ -2419,9 +2466,11 @@ struct timespec64 timestamp_truncate(struct timespec64 t, struct inode *inode)
>  	if (unlikely(t.tv_sec == sb->s_time_max || t.tv_sec == sb->s_time_min))
>  		t.tv_nsec = 0;
>  
> -	/* Avoid division in the common cases 1 ns and 1 s. */
> +	/* Avoid division in the common cases 1 ns, 2 ns and 1 s. */
>  	if (gran == 1)
>  		; /* nothing */
> +	else if (gran == 2)
> +		t.tv_nsec &= ~1L;

Is that trying to mask off I_CTIME_QUERIED?
If so, can we please use that constant as raw constants tend to be
confusing in the long run.