Re: [PATCH v6 2/7] fs: add infrastructure for multigrain timestamps

Jan Kara
2023-08-02T19:35:49Z

On Tue 25-07-23 10:58:15, 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>
> ---
> fs/inode.c | 98 ++++++++++++++++++++++++++++++++++++++----------------
> fs/stat.c | 41 +++++++++++++++++++++--
> include/linux/fs.h | 45 +++++++++++++++++++++++--
> 3 files changed, 151 insertions(+), 33 deletions(-)
>
> diff --git a/fs/inode.c b/fs/inode.c
> index d4ab92233062..369621e7faf5 100644
> --- a/fs/inode.c
> +++ b/fs/inode.c
> @@ -1919,6 +1919,21 @@ int inode_update_time(struct inode *inode, struct timespec64 *time, int flags)
> }
> EXPORT_SYMBOL(inode_update_time);
>
> +/**
> + * current_coarse_time - Return FS time
> + * @inode: inode.
> + *
> + * Return the current coarse-grained time truncated to the time
> + * granularity supported by the fs.
> + */
> +static struct timespec64 current_coarse_time(struct inode *inode)
> +{
> + struct timespec64 now;
> +
> + ktime_get_coarse_real_ts64(&now);
> + return timestamp_truncate(now, inode);
> +}
> +
> /**
> * atime_needs_update - update the access time
> * @path: the &struct path to update
> @@ -1952,7 +1967,7 @@ bool atime_needs_update(const struct path *path, struct inode *inode)
> if ((mnt->mnt_flags & MNT_NODIRATIME) && S_ISDIR(inode->i_mode))
> return false;
>
> - now = current_time(inode);
> + now = current_coarse_time(inode);
>
> if (!relatime_need_update(mnt, inode, now))
> return false;
> @@ -1986,7 +2001,7 @@ void touch_atime(const struct path *path)
> * We may also fail on filesystems that have the ability to make parts
> * of the fs read only, e.g. subvolumes in Btrfs.
> */
> - now = current_time(inode);
> + now = current_coarse_time(inode);
> inode_update_time(inode, &now, S_ATIME);
> __mnt_drop_write(mnt);
> skip_update:

There are also calls in fs/smb/client/file.c:cifs_readpage_worker() and in
fs/ocfs2/file.c:ocfs2_update_inode_atime() that should probably use
current_coarse_time() to avoid needless querying of fine grained
timestamps. But see below...

> @@ -2072,6 +2087,56 @@ 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.
> + */
> +static struct timespec64 current_mgtime(struct inode *inode)
> +{
> + struct timespec64 now;
> + 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);
> + } else {
> + struct timespec64 ctime;
> +
> + ktime_get_coarse_real_ts64(&now);
> +
> + /*
> + * If we've recently fetched a fine-grained timestamp
> + * then the coarse-grained one may still be earlier than the
> + * existing one. Just keep the existing ctime if so.
> + */
> + ctime = inode_get_ctime(inode);
> + if (timespec64_compare(&ctime, &now) > 0)
> + now = ctime;
> + }
> +
> + return timestamp_truncate(now, inode);
> +}
> +
> +/**
> + * current_time - Return timestamp suitable for ctime update
> + * @inode: inode to eventually be updated
> + *
> + * Return the current time, which is usually coarse-grained but may be fine
> + * grained if the filesystem uses multigrain timestamps and the existing
> + * ctime was queried since the last update.
> + */
> +struct timespec64 current_time(struct inode *inode)
> +{
> + if (is_mgtime(inode))
> + return current_mgtime(inode);
> + return current_coarse_time(inode);
> +}
> +EXPORT_SYMBOL(current_time);
> +

So if you modify current_time() to handle multigrain timestamps the code
will be still racy. In particular fill_mg_cmtime() can race with
inode_set_ctime_current() like:

fill_mg_cmtime() inode_set_ctime_current()
stat->mtime = inode->i_mtime;
stat->ctime.tv_sec = inode->__i_ctime.tv_sec;
now = current_time();
/* fetches coarse
* grained timestamp */
stat->ctime.tv_nsec = atomic_long_fetch_or(I_CTIME_QUERIED, pnsec) &
~I_CTIME_QUERIED;
inode_set_ctime(inode, now.tv_sec, now.tv_nsec);

and the information about a need for finegrained timestamp update gets
lost. So what I'd propose is to leave current_time() alone (just always
reporting coarse grained timestamps) and put all the magic into
inode_set_ctime_current() only. There we need something like:

struct timespec64 inode_set_ctime_current(struct inode *inode)
{
... variables ...

nsec = READ_ONCE(inode->__i_ctime.tv_nsec);
if (!(nsec & I_CTIME_QUERIED)) {
now = current_time(inode);

if (!is_gmtime(inode)) {
inode_set_ctime_to_ts(inode, now);
} else {
/*
* If we've recently fetched a fine-grained
* timestamp then the coarse-grained one may still
* be earlier than the existing one. Just keep the
* existing ctime if so.
*/
ctime = inode_get_ctime(inode);
if (timespec64_compare(&ctime, &now) > 0)
now = ctime;

/*
* Ctime updates are generally protected by inode
* lock but we could have raced with setting of
* I_CTIME_QUERIED flag.
*/
if (cmpxchg(&inode->__i_ctime.tv_nsec, nsec,
now.tv_nsec) != 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, now);

return now;
}

Honza

> static int inode_needs_update_time(struct inode *inode, struct timespec64 *now)
> {
> int sync_it = 0;
> @@ -2480,37 +2545,12 @@ 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);
> -
> - if (unlikely(!inode->i_sb)) {
> - WARN(1, "current_time() called with uninitialized super_block in the inode");
> - return 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->__i_ctime to the current value for the inode. Returns
> + * the current value that was assigned to __i_ctime.
> */
> struct timespec64 inode_set_ctime_current(struct inode *inode)
> {
> diff --git a/fs/stat.c b/fs/stat.c
> index 062f311b5386..51effd1c2bc2 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
> + * @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 fill_mg_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 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(request_mask, inode, stat);
> + } 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 42d1434cc427..a0bdbefbf293 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -1477,15 +1477,43 @@ static inline bool fsuidgid_has_mapping(struct super_block *sb,
> 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;
> }
>
> /**
> @@ -2261,6 +2289,7 @@ 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;
> @@ -2284,6 +2313,17 @@ 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));
> @@ -2919,6 +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(u32 request_mask, struct inode *inode, struct kstat *stat);
> 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