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

[Jeff Layton]

On Tue, 2023-04-25 at 07:47 +1000, NeilBrown wrote:
> 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@xxxxxxxxxx>
> > ---
> >  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)  ??
> 

I didn't realize that existed! Sure, I can make that change.

> > +	}
> > +
> > +	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 ??
> 
> 

The timestamp_truncate below will take care of it.

> > +		}
> > +	}
> > +
> > +	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) ???
> 

Even better. I'll revise it.

> > +#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?
> 

Sure. I'll add that in for the next one.

> > @@ -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);
>  ??
> 

I'm not sure I understand what you mean here. We want to check whether
SB_MULTIGRAIN_TS is set in the flags, and that s_time_gran > 1. The
latter is required so that we have space for the I_CTIME_QUERIED flag.

If SB_MULTIGRAIN_TS is set, but the s_time_gran is too low, we want to
throw a warning (since something is clearly wrong).


> > +}
> > +
> > +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!
> 

Thanks for taking a look! I think this has the potential to fix some
very long standing cache coherency issues in all NFS versions (v3 and
up).
-- 
Jeff Layton <jlayton@xxxxxxxxxx>