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

[Jeff Layton]

On Wed, 2023-04-26 at 08:53 +0200, Christian Brauner wrote:
> On Mon, Apr 24, 2023 at 11:11:02AM -0400, 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.
> > 
> > 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);
> > +	}
> > +
> > +	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;
> > +		}
> > +	}
> > +
> > +	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;
> 
> Is that trying to mask off I_CTIME_QUERIED?
> If so, can we please use that constant as raw constants tend to be
> confusing in the long run.

Sort of. In principle you could set s_time_gran to 2 without setting
SB_MULTIGRAIN_TS. In that case, would it be correct to use the flag
there?

In any case, I can certainly make it use that constant though if that's
what you'd prefer.
-- 
Jeff Layton <jlayton@xxxxxxxxxx>