Re: [PATCH v7 09/13] fs: add infrastructure for multigrain timestamps

Jan Kara
2023-08-08T17:18:58Z

On Mon 07-08-23 15:38:40, Jeff Layton wrote:
> The VFS always uses 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 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.
>
> POSIX generally mandates that when the the mtime changes, the ctime must
> also change. The kernel always stores normalized ctime values, so only
> the first 30 bits of the tv_nsec field are ever used.
>
> Use the 31st bit of the ctime tv_nsec field to indicate that something
> has queried the inode for the mtime or ctime. When this flag is set,
> on the next mtime or ctime update, the kernel will fetch a fine-grained
> timestamp instead of the usual coarse-grained one.
>
> Filesytems can opt into this behavior by setting the FS_MGTIME flag in
> the fstype. Filesystems that don't set this flag will continue to use
> coarse-grained timestamps.
>
> Later patches will convert individual filesystems to use the new
> infrastructure.
>
> Signed-off-by: Jeff Layton <jlayton@kernel.org>

Looks good to me. Feel free to add:

Reviewed-by: Jan Kara <jack@suse.cz>

Honza

> ---
> fs/inode.c | 82 ++++++++++++++++++++++++++++++++++++++++++++++++++++--
> fs/stat.c | 41 +++++++++++++++++++++++++--
> include/linux/fs.h | 46 ++++++++++++++++++++++++++++--
> 3 files changed, 162 insertions(+), 7 deletions(-)
>
> diff --git a/fs/inode.c b/fs/inode.c
> index e50d94a136fe..f55957ac80e6 100644
> --- a/fs/inode.c
> +++ b/fs/inode.c
> @@ -2118,10 +2118,52 @@ int file_remove_privs(struct file *file)
> }
> EXPORT_SYMBOL(file_remove_privs);
>
> +/**
> + * current_mgtime - 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_mgtime(struct inode *inode)
> +{
> + struct timespec64 now, ctime;
> + atomic_long_t *pnsec = (atomic_long_t *)&inode->__i_ctime.tv_nsec;
> + long nsec = atomic_long_read(pnsec);
> +
> + if (nsec & I_CTIME_QUERIED) {
> + ktime_get_real_ts64(&now);
> + return timestamp_truncate(now, inode);
> + }
> +
> + ktime_get_coarse_real_ts64(&now);
> + now = timestamp_truncate(now, inode);
> +
> + /*
> + * If we've recently fetched a fine-grained timestamp
> + * then the coarse-grained one may still be earlier than the
> + * existing ctime. Just keep the existing value if so.
> + */
> + ctime = inode_get_ctime(inode);
> + if (timespec64_compare(&ctime, &now) > 0)
> + now = ctime;
> +
> + return now;
> +}
> +EXPORT_SYMBOL(current_mgtime);
> +
> +static struct timespec64 current_ctime(struct inode *inode)
> +{
> + if (is_mgtime(inode))
> + return current_mgtime(inode);
> + return current_time(inode);
> +}
> +
> static int inode_needs_update_time(struct inode *inode)
> {
> int sync_it = 0;
> - struct timespec64 now = current_time(inode);
> + struct timespec64 now = current_ctime(inode);
> struct timespec64 ctime;
>
> /* First try to exhaust all avenues to not sync */
> @@ -2552,9 +2594,43 @@ EXPORT_SYMBOL(current_time);
> */
> struct timespec64 inode_set_ctime_current(struct inode *inode)
> {
> - struct timespec64 now = current_time(inode);
> + struct timespec64 now;
> + struct timespec64 ctime;
> +
> + ctime.tv_nsec = READ_ONCE(inode->__i_ctime.tv_nsec);
> + if (!(ctime.tv_nsec & I_CTIME_QUERIED)) {
> + now = current_time(inode);
>
> - inode_set_ctime(inode, now.tv_sec, now.tv_nsec);
> + /* Just copy it into place if it's not multigrain */
> + if (!is_mgtime(inode)) {
> + inode_set_ctime_to_ts(inode, now);
> + return now;
> + }
> +
> + /*
> + * If we've recently updated with a fine-grained timestamp,
> + * then the coarse-grained one may still be earlier than the
> + * existing ctime. Just keep the existing value if so.
> + */
> + ctime.tv_sec = inode->__i_ctime.tv_sec;
> + if (timespec64_compare(&ctime, &now) > 0)
> + return ctime;
> +
> + /*
> + * Ctime updates are usually protected by the inode_lock, but
> + * we can still race with someone setting the QUERIED flag.
> + * Try to swap the new nsec value into place. If it's changed
> + * in the interim, then just go with a fine-grained timestamp.
> + */
> + if (cmpxchg(&inode->__i_ctime.tv_nsec, ctime.tv_nsec,
> + now.tv_nsec) != ctime.tv_nsec)
> + goto fine_grained;
> + inode->__i_ctime.tv_sec = now.tv_sec;
> + return now;
> + }
> +fine_grained:
> + ktime_get_real_ts64(&now);
> + inode_set_ctime_to_ts(inode, timestamp_truncate(now, inode));
> return now;
> }
> EXPORT_SYMBOL(inode_set_ctime_current);
> diff --git a/fs/stat.c b/fs/stat.c
> index 7644e5997035..136711ae72fb 100644
> --- a/fs/stat.c
> +++ b/fs/stat.c
> @@ -26,6 +26,37 @@
> #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 use a fine-grained timestamp.
> + */
> +void fill_mg_cmtime(struct kstat *stat, u32 request_mask, struct inode *inode)
> +{
> + atomic_long_t *pnsec = (atomic_long_t *)&inode->__i_ctime.tv_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->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(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 +89,14 @@ void generic_fillattr(struct mnt_idmap *idmap, u32 request_mask,
> stat->rdev = inode->i_rdev;
> stat->size = i_size_read(inode);
> stat->atime = inode->i_atime;
> - stat->mtime = inode->i_mtime;
> - stat->ctime = inode_get_ctime(inode);
> +
> + if (is_mgtime(inode)) {
> + fill_mg_cmtime(stat, request_mask, inode);
> + } else {
> + stat->mtime = inode->i_mtime;
> + stat->ctime = inode_get_ctime(inode);
> + }
> +
> stat->blksize = i_blocksize(inode);
> stat->blocks = inode->i_blocks;
>
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index a83313f90fe3..455835d0e963 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -1474,18 +1474,47 @@ static inline bool fsuidgid_has_mapping(struct super_block *sb,
> kgid_has_mapping(fs_userns, kgid);
> }
>
> +struct timespec64 current_mgtime(struct inode *inode);
> struct timespec64 current_time(struct inode *inode);
> struct timespec64 inode_set_ctime_current(struct inode *inode);
>
> +/*
> + * 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.
> + *
> + * The kernel always keeps normalized struct timespec64 values in the ctime,
> + * which means that only the first 30 bits of the value are used. Use the
> + * 31st bit of the ctime's tv_nsec field as a flag to indicate that the value
> + * has been queried since it was last updated.
> + */
> +#define I_CTIME_QUERIED (1L<<30)
> +
> /**
> * inode_get_ctime - fetch the current ctime from the inode
> * @inode: inode from which to fetch ctime
> *
> - * Grab the current ctime from the inode and return it.
> + * Grab the current ctime tv_nsec field from the inode, mask off the
> + * I_CTIME_QUERIED flag and return it. This is mostly intended for use by
> + * internal consumers of the ctime that aren't concerned with ensuring a
> + * fine-grained update on the next change (e.g. when preparing to store
> + * the value in the backing store for later retrieval).
> + *
> + * This is safe to call regardless of whether the underlying filesystem
> + * is using multigrain timestamps.
> */
> static inline struct timespec64 inode_get_ctime(const struct inode *inode)
> {
> - return inode->__i_ctime;
> + struct timespec64 ctime;
> +
> + ctime.tv_sec = inode->__i_ctime.tv_sec;
> + ctime.tv_nsec = inode->__i_ctime.tv_nsec & ~I_CTIME_QUERIED;
> +
> + return ctime;
> }
>
> /**
> @@ -2259,6 +2288,7 @@ 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;
> @@ -2282,6 +2312,17 @@ 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));
> @@ -2918,6 +2959,7 @@ 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);
>
> --
> 2.41.0
>
--
Jan Kara <jack@suse.com>
SUSE Labs, CR