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

NeilBrown
2023-04-24T21:48:13Z

On Tue, 25 Apr 2023, 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.

This assumes that any s_time_gran value greater then 1, is even. This is
currently true in practice (it is always a power of 10 I think).
But should we have a WARN_ON_ONCE() somewhere just in case?

>
> 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);

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 = inode->i_ctime;
> +
> + if (timespec64_compare(&ctime, &now) > 0)
> + now = ctime;

I think this ctime could have the I_CTIME_QUERIED bit set. We probably
don't want that ??

> + }
> + }
> +
> + 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;
> else if (gran == NSEC_PER_SEC)
> t.tv_nsec = 0;
> else if (gran > 1 && gran < NSEC_PER_SEC)
> diff --git a/fs/stat.c b/fs/stat.c
> index 7c238da22ef0..67b56daf9663 100644
> --- a/fs/stat.c
> +++ b/fs/stat.c
> @@ -26,6 +26,30 @@
> #include "internal.h"
> #include "mount.h"
>
> +/**
> + * generic_fill_multigrain_cmtime - Fill in the mtime and ctime and flag ctime as QUERIED
> + * @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(struct inode *inode, struct kstat *stat)
> +{
> + atomic_long_t *pnsec = (atomic_long_t *)&inode->i_ctime.tv_nsec;
> +
> + 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);
> +
> /**
> * generic_fillattr - Fill in the basic attributes from the inode struct
> * @idmap: idmap of the mount the inode was found from
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index c85916e9f7db..e6dd3ce051ef 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -1059,21 +1059,22 @@ extern int send_sigurg(struct fown_struct *fown);
> * sb->s_flags. Note that these mirror the equivalent MS_* flags where
> * represented in both.
> */
> -#define SB_RDONLY 1 /* Mount read-only */
> -#define SB_NOSUID 2 /* Ignore suid and sgid bits */
> -#define SB_NODEV 4 /* Disallow access to device special files */
> -#define SB_NOEXEC 8 /* Disallow program execution */
> -#define SB_SYNCHRONOUS 16 /* Writes are synced at once */
> -#define SB_MANDLOCK 64 /* Allow mandatory locks on an FS */
> -#define SB_DIRSYNC 128 /* Directory modifications are synchronous */
> -#define SB_NOATIME 1024 /* Do not update access times. */
> -#define SB_NODIRATIME 2048 /* Do not update directory access times */
> -#define SB_SILENT 32768
> -#define SB_POSIXACL (1<<16) /* VFS does not apply the umask */
> -#define SB_INLINECRYPT (1<<17) /* Use blk-crypto for encrypted files */
> -#define SB_KERNMOUNT (1<<22) /* this is a kern_mount call */
> -#define SB_I_VERSION (1<<23) /* Update inode I_version field */
> -#define SB_LAZYTIME (1<<25) /* Update the on-disk [acm]times lazily */
> +#define SB_RDONLY (1<<0) /* Mount read-only */

BIT(0) ???

> +#define SB_NOSUID (1<<1) /* Ignore suid and sgid bits */

BIT(1) ??

> +#define SB_NODEV (1<<2) /* Disallow access to device special files */
> +#define SB_NOEXEC (1<<3) /* Disallow program execution */
> +#define SB_SYNCHRONOUS (1<<4) /* Writes are synced at once */
> +#define SB_MANDLOCK (1<<6) /* Allow mandatory locks on an FS */
> +#define SB_DIRSYNC (1<<7) /* Directory modifications are synchronous */
> +#define SB_NOATIME (1<<10) /* Do not update access times. */
> +#define SB_NODIRATIME (1<<11) /* Do not update directory access times */
> +#define SB_SILENT (1<<15)
> +#define SB_POSIXACL (1<<16) /* VFS does not apply the umask */
> +#define SB_INLINECRYPT (1<<17) /* Use blk-crypto for encrypted files */
> +#define SB_KERNMOUNT (1<<22) /* this is a kern_mount call */
> +#define SB_I_VERSION (1<<23) /* Update inode I_version field */
> +#define SB_MULTIGRAIN_TS (1<<24) /* Use multigrain c/mtimes */
> +#define SB_LAZYTIME (1<<25) /* Update the on-disk [acm]times lazily */
>
> /* These sb flags are internal to the kernel */
> #define SB_SUBMOUNT (1<<26)

Why not align this one too?

> @@ -1457,7 +1458,8 @@ static inline bool fsuidgid_has_mapping(struct super_block *sb,
> kgid_has_mapping(fs_userns, kgid);
> }
>
> -extern struct timespec64 current_time(struct inode *inode);
> +struct timespec64 current_time(struct inode *inode);
> +struct timespec64 current_ctime(struct inode *inode);
>
> /*
> * Snapshotting support.
> @@ -2171,8 +2173,31 @@ enum file_time_flags {
> S_VERSION = 8,
> };
>
> -extern bool atime_needs_update(const struct path *, struct inode *);
> -extern void touch_atime(const struct path *);
> +/*
> + * Multigrain timestamps
> + *
> + * Conditionally use fine-grained ctime and mtime timestamps
> + *
> + * When s_time_gran is >1, and SB_MULTIGRAIN_TS is set, use the lowest-order bit
> + * in the tv_nsec field as a flag to indicate that the value was recently queried
> + * and that the next update should use a fine-grained timestamp.
> + */
> +#define I_CTIME_QUERIED 1L
> +
> +static inline bool is_multigrain_ts(struct inode *inode)
> +{
> + struct super_block *sb = inode->i_sb;
> +
> + /*
> + * Warn if someone sets SB_MULTIGRAIN_TS, but doesn't turn down the ts
> + * granularity.
> + */
> + return (sb->s_flags & SB_MULTIGRAIN_TS) &&
> + !WARN_ON_ONCE(sb->s_time_gran == 1);

Maybe
!WARN_ON_ONCE(sb->s_time_gran & SB_MULTIGRAIN_TS);
??

> +}
> +
> +bool atime_needs_update(const struct path *, struct inode *);
> +void touch_atime(const struct path *);
> int inode_update_time(struct inode *inode, struct timespec64 *time, int flags);
>
> static inline void file_accessed(struct file *file)
> @@ -2838,6 +2863,7 @@ 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 generic_fill_multigrain_cmtime(struct inode *inode, struct kstat *stat);
> void generic_fillattr(struct mnt_idmap *, 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);
> --
> 2.40.0
>
>

Looks generally sensible, thanks!

NeilBrown