Re: [PATCH 01/10] fs: turn inode ctime fields into a single ktime_t

Darrick J. Wong
2024-07-01T22:49:42Z

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@kernel.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
>
>