Re: [PATCH v3 1/9] fs: add infrastructure for multigrain timestamps

Uros Bizjak
2024-07-08T15:32:52Z

On 5. 07. 24 19:02, Jeff Layton wrote:
> The VFS has always used coarse-grained timestamps when updating 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. A lot of changes
> can happen in a jiffy, so timestamps aren't sufficient to help the
> client decide when to invalidate the cache. 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 with timestamps (e.g backup
> applications).
>
> If we were to always use fine-grained timestamps, that would improve the
> situation, but that becomes rather expensive, as the underlying
> filesystem would 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. Use the (unused) top bit in inode->i_ctime_nsec
> as a flag that indicates whether the current timestamps have been
> queried via stat() or the like. When it's set, we allow the kernel to
> use a fine-grained timestamp iff it's necessary to make the ctime show
> a different value.
>
> This solves the problem of being able to distinguish the timestamp
> between updates, but introduces a new problem: it's now possible for a
> file being changed to get a fine-grained timestamp. A file that is
> altered just a bit later can then get a coarse-grained one that appears
> older than the earlier fine-grained time. This violates timestamp
> ordering guarantees.
>
> To remedy this, keep a global monotonic ktime_t value that acts as a
> timestamp floor. When we go to stamp a file, we first get the latter of
> the current floor value and the current coarse-grained time. If the
> inode ctime hasn't been queried then we just attempt to stamp it with
> that value.
>
> If it has been queried, then first see whether the current coarse time
> is later than the existing ctime. If it is, then we accept that value.
> If it isn't, then we get a fine-grained time and try to swap that into
> the global floor. Whether that succeeds or fails, we take the resulting
> floor time, convert it to realtime and try to swap that into the ctime.
>
> We take the result of the ctime swap whether it succeeds or fails, since
> either is just as valid.
>
> Filesystems can opt into this by setting the FS_MGTIME fstype flag.
> Others should be unaffected (other than being subject to the same floor
> value as multigrain filesystems).
>
> Signed-off-by: Jeff Layton <jlayton@kernel.org>
> ---
> fs/inode.c | 172 ++++++++++++++++++++++++++++++++++++++++++++---------
> fs/stat.c | 36 ++++++++++-
> include/linux/fs.h | 34 ++++++++---
> 3 files changed, 205 insertions(+), 37 deletions(-)
>
> diff --git a/fs/inode.c b/fs/inode.c
> index f356fe2ec2b6..844ff0750959 100644
> --- a/fs/inode.c
> +++ b/fs/inode.c
> @@ -60,6 +60,12 @@ static unsigned int i_hash_shift __ro_after_init;
> static struct hlist_head *inode_hashtable __ro_after_init;
> static __cacheline_aligned_in_smp DEFINE_SPINLOCK(inode_hash_lock);
>
> +/*
> + * This represents the latest time that we have handed out as a
> + * timestamp on the system. Tracked as a MONOTONIC value, and
> + * converted to the realtime clock on an as-needed basis.
> + */
> +static __cacheline_aligned_in_smp ktime_t ctime_floor;
> /*
> * Empty aops. Can be used for the cases where the user does not
> * define any of the address_space operations.
> @@ -2127,19 +2133,72 @@ int file_remove_privs(struct file *file)
> }
> EXPORT_SYMBOL(file_remove_privs);
>
> +/**
> + * coarse_ctime - return the current coarse-grained time
> + * @floor: current ctime_floor value
> + *
> + * Get the coarse-grained time, and then determine whether to
> + * return it or the current floor value. Returns the later of the
> + * floor and coarse grained timestamps, converted to realtime
> + * clock value.
> + */
> +static ktime_t coarse_ctime(ktime_t floor)
> +{
> + ktime_t coarse = ktime_get_coarse();
> +
> + /* If coarse time is already newer, return that */
> + if (!ktime_after(floor, coarse))
> + return ktime_mono_to_real(coarse);
> + return ktime_mono_to_real(floor);
> +}
> +
> +/**
> + * current_time - 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. If the ctime is flagged
> + * as having been QUERIED, get a fine-grained timestamp.
> + */
> +struct timespec64 current_time(struct inode *inode)
> +{
> + ktime_t floor = smp_load_acquire(&ctime_floor);
> + ktime_t now = coarse_ctime(floor);
> + struct timespec64 now_ts = ktime_to_timespec64(now);
> + u32 cns;
> +
> + if (!is_mgtime(inode))
> + goto out;
> +
> + /* If nothing has queried it, then coarse time is fine */
> + cns = smp_load_acquire(&inode->i_ctime_nsec);
> + if (cns & I_CTIME_QUERIED) {
> + /*
> + * If there is no apparent change, then
> + * get a fine-grained timestamp.
> + */
> + if (now_ts.tv_nsec == (cns & ~I_CTIME_QUERIED))
> + ktime_get_real_ts64(&now_ts);
> + }
> +out:
> + return timestamp_truncate(now_ts, inode);
> +}
> +EXPORT_SYMBOL(current_time);
> +
> static int inode_needs_update_time(struct inode *inode)
> {
> + struct timespec64 now, ts;
> int sync_it = 0;
> - struct timespec64 now = current_time(inode);
> - struct timespec64 ts;
>
> /* First try to exhaust all avenues to not sync */
> if (IS_NOCMTIME(inode))
> return 0;
>
> + now = current_time(inode);
> +
> ts = inode_get_mtime(inode);
> if (!timespec64_equal(&ts, &now))
> - sync_it = S_MTIME;
> + sync_it |= S_MTIME;
>
> ts = inode_get_ctime(inode);
> if (!timespec64_equal(&ts, &now))
> @@ -2507,6 +2566,14 @@ void inode_nohighmem(struct inode *inode)
> }
> EXPORT_SYMBOL(inode_nohighmem);
>
> +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 & ~I_CTIME_QUERIED;
> + return ts;
> +}
> +EXPORT_SYMBOL(inode_set_ctime_to_ts);
> +
> /**
> * timestamp_truncate - Truncate timespec to a granularity
> * @t: Timespec
> @@ -2538,38 +2605,89 @@ struct timespec64 timestamp_truncate(struct timespec64 t, struct inode *inode)
> }
> EXPORT_SYMBOL(timestamp_truncate);
>
> -/**
> - * current_time - Return FS time
> - * @inode: inode.
> - *
> - * Return the current time truncated to the time granularity supported by
> - * the fs.
> - *
> - * Note that inode and inode->sb cannot be NULL.
> - * Otherwise, the function warns and returns time without truncation.
> - */
> -struct timespec64 current_time(struct inode *inode)
> -{
> - struct timespec64 now;
> -
> - ktime_get_coarse_real_ts64(&now);
> - return timestamp_truncate(now, inode);
> -}
> -EXPORT_SYMBOL(current_time);
> -
> /**
> * inode_set_ctime_current - set the ctime to current_time
> * @inode: inode
> *
> - * Set the inode->i_ctime to the current value for the inode. Returns
> - * the current value that was assigned to i_ctime.
> + * Set the inode's ctime to the current value for the inode. Returns
> + * the current value that was assigned. If this is a not multigrain inode,
> + * then we just set it to whatever the coarse_ctime is.
> + *
> + * If it is multigrain, then we first see if the coarse-grained
> + * timestamp is distinct from what we have. If so, then we'll just use
> + * that. If we have to get a fine-grained timestamp, then do so, and
> + * try to swap it into the floor. We accept the new floor value
> + * regardless of the outcome of the cmpxchg. After that, we try to
> + * swap the new value into i_ctime_nsec. Again, we take the resulting
> + * ctime, regardless of the outcome of the swap.
> */
> struct timespec64 inode_set_ctime_current(struct inode *inode)
> {
> - struct timespec64 now = current_time(inode);
> + ktime_t now, floor = smp_load_acquire(&ctime_floor);
> + struct timespec64 now_ts;
> + u32 cns, cur;
> +
> + now = coarse_ctime(floor);
> +
> + /* Just return that if this is not a multigrain fs */
> + if (!is_mgtime(inode)) {
> + now_ts = ktime_to_timespec64(now);
> + inode_set_ctime_to_ts(inode, now_ts);
> + goto out;
> + }
>
> - inode_set_ctime_to_ts(inode, now);
> - return now;
> + /*
> + * We only need a fine-grained time if someone has queried it,
> + * and the current coarse grained time isn't later than what's
> + * already there.
> + */
> + cns = smp_load_acquire(&inode->i_ctime_nsec);
> + if (cns & I_CTIME_QUERIED) {
> + ktime_t ctime = ktime_set(inode->i_ctime_sec, cns & ~I_CTIME_QUERIED);
> +
> + if (!ktime_after(now, ctime)) {
> + ktime_t old, fine;
> +
> + /* Get a fine-grained time */
> + fine = ktime_get();
> +
> + /*
> + * If the cmpxchg works, we take the new floor value. If
> + * not, then that means that someone else changed it after we
> + * fetched it but before we got here. That value is just
> + * as good, so keep it.
> + */
> + old = cmpxchg(&ctime_floor, floor, fine);
> + if (old != floor)
> + fine = old;

The above can be rewritten using try_cmpxchg() to produce slightly
better (and faster) code:

old = floor;
if (!try_cmpxchg(&ctime_floor, &old, fine)
fine = old;

try_cmpxchg() updates old on failure.

> + now = ktime_mono_to_real(fine);
> + }
> + }
> + now_ts = ktime_to_timespec64(now);
> +retry:
> + /* Try to swap the nsec value into place. */
> + cur = cmpxchg(&inode->i_ctime_nsec, cns, now_ts.tv_nsec);
> +
> + /* If swap occurred, then we're (mostly) done */
> + if (cur == cns) {
> + inode->i_ctime_sec = now_ts.tv_sec;
> + } else {

The above can be rewritten to:

retry:
/* Try to swap the nsec value into place. */
cur = cns;
if (try_cmpxchg(&inode->i_ctime_ns, &cur, now_ts.tv_nsec) {
/* If swap occurred, then we're (mostly) done */
inode->i_ctime_sec = now_ts.tv_sec;
} else {
...

in the else arm, cur will have the return value from CMPXCHG
instruction, as in the original source. Please note that try_cmpxchg()
updates cur only on failure.

For x86 targets, the proposed form will avoid extra CMP instruction and
will use zero-flag result of the CMPXCHG instruction instead.

Best regards,
Uros.

> + /*
> + * Was the change due to someone marking the old ctime QUERIED?
> + * If so then retry the swap. This can only happen once since
> + * the only way to clear I_CTIME_QUERIED is to stamp the inode
> + * with a new ctime.
> + */
> + if (!(cns & I_CTIME_QUERIED) && (cns | I_CTIME_QUERIED) == cur) {
> + cns = cur;
> + goto retry;
> + }
> + /* Otherwise, keep the existing ctime */
> + now_ts.tv_sec = inode->i_ctime_sec;
> + now_ts.tv_nsec = cur & ~I_CTIME_QUERIED;
> + }
> +out:
> + return timestamp_truncate(now_ts, inode);
> }
> EXPORT_SYMBOL(inode_set_ctime_current);
>
> diff --git a/fs/stat.c b/fs/stat.c
> index 6f65b3456cad..df7fdd3afed9 100644
> --- a/fs/stat.c
> +++ b/fs/stat.c
> @@ -26,6 +26,32 @@
> #include "internal.h"
> #include "mount.h"
>
> +/**
> + * fill_mg_cmtime - Fill in the mtime and ctime and flag ctime as QUERIED
> + * @stat: where to store the resulting values
> + * @request_mask: STATX_* values requested
> + * @inode: inode from which to grab the c/mtime
> + *
> + * 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 ensure a distinct timestamp.
> + */
> +void fill_mg_cmtime(struct kstat *stat, u32 request_mask, struct inode *inode)
> +{
> + atomic_t *pcn = (atomic_t *)&inode->i_ctime_nsec;
> +
> + /* If neither time was requested, then don't report them */
> + if (!(request_mask & (STATX_CTIME|STATX_MTIME))) {
> + stat->result_mask &= ~(STATX_CTIME|STATX_MTIME);
> + return;
> + }
> +
> + stat->mtime = inode_get_mtime(inode);
> + stat->ctime.tv_sec = inode->i_ctime_sec;
> + stat->ctime.tv_nsec = ((u32)atomic_fetch_or(I_CTIME_QUERIED, pcn)) & ~I_CTIME_QUERIED;
> +}
> +EXPORT_SYMBOL(fill_mg_cmtime);
> +
> /**
> * generic_fillattr - Fill in the basic attributes from the inode struct
> * @idmap: idmap of the mount the inode was found from
> @@ -58,8 +84,14 @@ void generic_fillattr(struct mnt_idmap *idmap, u32 request_mask,
> stat->rdev = inode->i_rdev;
> stat->size = i_size_read(inode);
> stat->atime = inode_get_atime(inode);
> - stat->mtime = inode_get_mtime(inode);
> - stat->ctime = inode_get_ctime(inode);
> +
> + if (is_mgtime(inode)) {
> + fill_mg_cmtime(stat, request_mask, inode);
> + } else {
> + stat->ctime = inode_get_ctime(inode);
> + stat->mtime = inode_get_mtime(inode);
> + }
> +
> stat->blksize = i_blocksize(inode);
> stat->blocks = inode->i_blocks;
>
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index dc9f9c4b2572..f873f6c58669 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -1608,6 +1608,17 @@ static inline struct timespec64 inode_set_mtime(struct inode *inode,
> return inode_set_mtime_to_ts(inode, ts);
> }
>
> +/*
> + * Multigrain timestamps
> + *
> + * Conditionally use fine-grained ctime and mtime timestamps when there
> + * are users actively observing them via getattr. The primary use-case
> + * for this is NFS clients that use the ctime to distinguish between
> + * different states of the file, and that are often fooled by multiple
> + * operations that occur in the same coarse-grained timer tick.
> + */
> +#define I_CTIME_QUERIED ((u32)BIT(31))
> +
> static inline time64_t inode_get_ctime_sec(const struct inode *inode)
> {
> return inode->i_ctime_sec;
> @@ -1615,7 +1626,7 @@ static inline time64_t inode_get_ctime_sec(const struct inode *inode)
>
> static inline long inode_get_ctime_nsec(const struct inode *inode)
> {
> - return inode->i_ctime_nsec;
> + return inode->i_ctime_nsec & ~I_CTIME_QUERIED;
> }
>
> static inline struct timespec64 inode_get_ctime(const struct inode *inode)
> @@ -1626,13 +1637,7 @@ static inline struct timespec64 inode_get_ctime(const struct inode *inode)
> return ts;
> }
>
> -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;
> - return ts;
> -}
> +struct timespec64 inode_set_ctime_to_ts(struct inode *inode, struct timespec64 ts);
>
> /**
> * inode_set_ctime - set the ctime in the inode
> @@ -2490,6 +2495,7 @@ struct file_system_type {
> #define FS_USERNS_MOUNT 8 /* Can be mounted by userns root */
> #define FS_DISALLOW_NOTIFY_PERM 16 /* Disable fanotify permission events */
> #define FS_ALLOW_IDMAP 32 /* FS has been updated to handle vfs idmappings. */
> +#define FS_MGTIME 64 /* FS uses multigrain timestamps */
> #define FS_RENAME_DOES_D_MOVE 32768 /* FS will handle d_move() during rename() internally. */
> int (*init_fs_context)(struct fs_context *);
> const struct fs_parameter_spec *parameters;
> @@ -2513,6 +2519,17 @@ struct file_system_type {
>
> #define MODULE_ALIAS_FS(NAME) MODULE_ALIAS("fs-" NAME)
>
> +/**
> + * is_mgtime: is this inode using multigrain timestamps
> + * @inode: inode to test for multigrain timestamps
> + *
> + * Return true if the inode uses multigrain timestamps, false otherwise.
> + */
> +static inline bool is_mgtime(const struct inode *inode)
> +{
> + return inode->i_sb->s_type->fs_flags & FS_MGTIME;
> +}
> +
> extern struct dentry *mount_bdev(struct file_system_type *fs_type,
> int flags, const char *dev_name, void *data,
> int (*fill_super)(struct super_block *, void *, int));
> @@ -3252,6 +3269,7 @@ extern void page_put_link(void *);
> extern int page_symlink(struct inode *inode, const char *symname, int len);
> extern const struct inode_operations page_symlink_inode_operations;
> extern void kfree_link(void *);
> +void fill_mg_cmtime(struct kstat *stat, u32 request_mask, struct inode *inode);
> void generic_fillattr(struct mnt_idmap *, u32, struct inode *, struct kstat *);
> void generic_fill_statx_attr(struct inode *inode, struct kstat *stat);
> extern int vfs_getattr_nosec(const struct path *, struct kstat *, u32, unsigned int);
>