linux-kernel - Re: [PATCH 01/10] fs: turn inode ctime fields into a single ktime

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Date: Mon, 1 Jul 2024 15:49:41 -0700
From: "Darrick J. Wong" <djwong@...nel.org>
To: Jeff Layton <jlayton@...nel.org>
Cc: Alexander Viro <viro@...iv.linux.org.uk>,
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
	Steven Rostedt <rostedt@...dmis.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Chandan Babu R <chandan.babu@...cle.com>,
	Theodore Ts'o <tytso@....edu>,
	Andreas Dilger <adilger.kernel@...ger.ca>, Chris Mason <clm@...com>,
	Josef Bacik <josef@...icpanda.com>, David Sterba <dsterba@...e.com>,
	Hugh Dickins <hughd@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>, kernel-team@...com,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-trace-kernel@...r.kernel.org, linux-xfs@...r.kernel.org,
	linux-ext4@...r.kernel.org, linux-btrfs@...r.kernel.org,
	linux-mm@...ck.org, linux-nfs@...r.kernel.org
Subject: Re: [PATCH 01/10] fs: turn inode ctime fields into a single ktime_t

On Wed, Jun 26, 2024 at 09:00:21PM -0400, Jeff Layton wrote:
> The ctime is not settable to arbitrary values. It always comes from the
> system clock, so we'll never stamp an inode with a value that can't be
> represented there. If we disregard people setting their system clock
> past the year 2262, there is no reason we can't replace the ctime fields
> with a ktime_t.
> 
> Switch the ctime fields to a single ktime_t. Move the i_generation down
> above i_fsnotify_mask and then move the i_version into the resulting 8
> byte hole. This shrinks struct inode by 8 bytes total, and should
> improve the cache footprint as the i_version and ctime are usually
> updated together.
> 
> The one downside I can see to switching to a ktime_t is that if someone
> has a filesystem with files on it that has ctimes outside the ktime_t
> range (before ~1678 AD or after ~2262 AD), we won't be able to display
> them properly in stat() without some special treatment in the
> filesystem. The operating assumption here is that that is not a
> practical problem.

What happens if a filesystem with the ability to store ctimes beyond
whatever ktime_t supports (AFAICT 2^63-1 nanonseconds on either side of
the Unix epoch)?  I think the behavior with your patch is that ktime_set
clamps the ctime on iget because the kernel can't handle it?

It's a little surprising that the ctime will suddenly jump back in time
to 2262, but maybe you're right that nobody will notice or care? ;)

--D

> 
> Signed-off-by: Jeff Layton <jlayton@...nel.org>
> ---
>  include/linux/fs.h | 26 +++++++++++---------------
>  1 file changed, 11 insertions(+), 15 deletions(-)
> 
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 5ff362277834..5139dec085f2 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -662,11 +662,10 @@ struct inode {
>  	loff_t			i_size;
>  	time64_t		i_atime_sec;
>  	time64_t		i_mtime_sec;
> -	time64_t		i_ctime_sec;
>  	u32			i_atime_nsec;
>  	u32			i_mtime_nsec;
> -	u32			i_ctime_nsec;
> -	u32			i_generation;
> +	ktime_t			__i_ctime;
> +	atomic64_t		i_version;
>  	spinlock_t		i_lock;	/* i_blocks, i_bytes, maybe i_size */
>  	unsigned short          i_bytes;
>  	u8			i_blkbits;
> @@ -701,7 +700,6 @@ struct inode {
>  		struct hlist_head	i_dentry;
>  		struct rcu_head		i_rcu;
>  	};
> -	atomic64_t		i_version;
>  	atomic64_t		i_sequence; /* see futex */
>  	atomic_t		i_count;
>  	atomic_t		i_dio_count;
> @@ -724,6 +722,8 @@ struct inode {
>  	};
>  
>  
> +	u32			i_generation;
> +
>  #ifdef CONFIG_FSNOTIFY
>  	__u32			i_fsnotify_mask; /* all events this inode cares about */
>  	/* 32-bit hole reserved for expanding i_fsnotify_mask */
> @@ -1608,29 +1608,25 @@ static inline struct timespec64 inode_set_mtime(struct inode *inode,
>  	return inode_set_mtime_to_ts(inode, ts);
>  }
>  
> -static inline time64_t inode_get_ctime_sec(const struct inode *inode)
> +static inline struct timespec64 inode_get_ctime(const struct inode *inode)
>  {
> -	return inode->i_ctime_sec;
> +	return ktime_to_timespec64(inode->__i_ctime);
>  }
>  
> -static inline long inode_get_ctime_nsec(const struct inode *inode)
> +static inline time64_t inode_get_ctime_sec(const struct inode *inode)
>  {
> -	return inode->i_ctime_nsec;
> +	return inode_get_ctime(inode).tv_sec;
>  }
>  
> -static inline struct timespec64 inode_get_ctime(const struct inode *inode)
> +static inline long inode_get_ctime_nsec(const struct inode *inode)
>  {
> -	struct timespec64 ts = { .tv_sec  = inode_get_ctime_sec(inode),
> -				 .tv_nsec = inode_get_ctime_nsec(inode) };
> -
> -	return ts;
> +	return inode_get_ctime(inode).tv_nsec;
>  }
>  
>  static inline struct timespec64 inode_set_ctime_to_ts(struct inode *inode,
>  						      struct timespec64 ts)
>  {
> -	inode->i_ctime_sec = ts.tv_sec;
> -	inode->i_ctime_nsec = ts.tv_nsec;
> +	inode->__i_ctime = ktime_set(ts.tv_sec, ts.tv_nsec);
>  	return ts;
>  }
>  
> 
> -- 
> 2.45.2
> 
>