On Wed, May 03, 2023 at 10:20:32AM -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 metadata updates, down to around 1
> 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, 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.
>
> The kernel always stores normalized ctime values, so only the first 30
> bits of the tv_nsec field are ever used. Whenever the mtime changes, the
> ctime must also change.
>
> Use the 31st bit of the tv_nsec field to indicate that something has
> queried the inode for the i_mtime or i_ctime. When this flag is set, on
> the next timestamp update, the kernel can fetch a fine-grained timestamp
> instead of the usual coarse-grained one.
>
> This patch adds the infrastructure this scheme. Filesytems can opt
> into it by setting the FS_MULTIGRAIN_TS flag in the fstype.
>
> Later patches will convert individual filesystems over to use it.
>
> Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
> ---
> fs/inode.c | 52 ++++++++++++++++++++++++++++++++++++---
> fs/stat.c | 32 ++++++++++++++++++++++++
> include/linux/fs.h | 61 +++++++++++++++++++++++++++++++++++++++++++++-
> 3 files changed, 141 insertions(+), 4 deletions(-)
>
> diff --git a/fs/inode.c b/fs/inode.c
> index 4558dc2f1355..7f6189961d6a 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;
>
> /* First try to exhaust all avenues to not sync */
> if (IS_NOCMTIME(inode))
> @@ -2038,7 +2039,8 @@ 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))
> + ctime = ctime_peek(inode);
> + if (!timespec64_equal(&ctime, now))
> sync_it |= S_CTIME;
>
> if (IS_I_VERSION(inode) && inode_iversion_need_inc(inode))
> @@ -2062,6 +2064,50 @@ static int __file_update_time(struct file *file, struct timespec64 *now,
> return ret;
> }
>
> +/**
> + * current_ctime - Return FS time (possibly fine-grained)
> + * @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 ctime is flagged as having been
> + * QUERIED, get a fine-grained timestamp.
> + */
> +struct timespec64 current_ctime(struct inode *inode)
> +{
> + bool multigrain = is_multigrain_ts(inode);
> + struct timespec64 now;
> + long nsec = 0;
> +
> + if (multigrain) {
> + atomic_long_t *pnsec = (atomic_long_t *)&inode->i_ctime.tv_nsec;
> +
> + nsec = atomic_long_fetch_andnot(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 = ctime_peek(inode);
> +
> + if (timespec64_compare(&ctime, &now) > 0)
> + now = ctime;
> + }
> + }
> +
> + return timestamp_truncate(now, inode);
> +}
> +EXPORT_SYMBOL(current_ctime);
I can't help but think this is easier to read/follow when structured
to separate multigrain vs coarse logic completely like so:
struct timespec64 current_ctime(struct inode *inode)
{
struct timespec64 now, ctime;
long nsec;
if (!is_multigrain_ts(inode)) {
ktime_get_coarse_real_ts64(&now);
goto out_truncate;
}
nsec = atomic_long_fetch_andnot(I_CTIME_QUERIED,
(atomic_long_t *)&inode->i_ctime.tv_nsec);
if (nsec & I_CTIME_QUERIED) {
ktime_get_real_ts64(&now);
goto out_truncate;
}
/*
* 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.
*/
ktime_get_coarse_real_ts64(&now);
ctime = ctime_peek(inode);
if (timespec64_compare(&ctime, &now) > 0)
now = ctime;
out_truncate:
return timestamp_truncate(now, inode);
}
> diff --git a/fs/stat.c b/fs/stat.c
> index 7c238da22ef0..11a7e277f53e 100644
> --- a/fs/stat.c
> +++ b/fs/stat.c
> @@ -26,6 +26,38 @@
> #include "internal.h"
> #include "mount.h"
>
> +/**
> + * generic_fill_multigrain_cmtime - Fill in the mtime and ctime and flag ctime as QUERIED
> + * @request_mask: STATX_* values requested
> + * @inode: inode from which to grab the c/mtime
> + * @stat: where to store the resulting values
> + *
> + * Given @inode, grab the ctime and mtime out if it and store the result
> + * in @stat. When fetching the value, flag it as queried so the next write
> + * will use a fine-grained timestamp.
> + */
> +void generic_fill_multigrain_cmtime(u32 request_mask,struct inode *inode,
> + struct kstat *stat)
> +{
> + atomic_long_t *pnsec = (atomic_long_t *)&inode->i_ctime.tv_nsec;
> +
> + /* If neither time was requested, then just don't report it */
> + if (!(request_mask & (STATX_CTIME|STATX_MTIME))) {
> + stat->result_mask &= ~(STATX_CTIME|STATX_MTIME);
> + return;
> + }
> +
> + stat->mtime = inode->i_mtime;
> + stat->ctime.tv_sec = inode->i_ctime.tv_sec;
> + /*
> + * Atomically set the QUERIED flag and fetch the new value with
> + * the flag masked off.
> + */
> + stat->ctime.tv_nsec = atomic_long_fetch_or(I_CTIME_QUERIED, pnsec) &
> + ~I_CTIME_QUERIED;
> +}
> +EXPORT_SYMBOL(generic_fill_multigrain_cmtime);
Hmmm - why not just have a generic_fill_cmtime() function that hides
multigrain behaviour from all the statx callers?
Cheers,
Dave.
--
Dave Chinner
david@xxxxxxxxxxxxx
