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Subject: Re: [PATCH v2 1/3] fs: add infrastructure for multigrain inode
 i_m/ctime
From:   Jeff Layton <jlayton@kernel.org>
To:     Christian Brauner <brauner@kernel.org>
Cc:     Alexander Viro <viro@zeniv.linux.org.uk>,
        "Darrick J. Wong" <djwong@kernel.org>,
        Hugh Dickins <hughd@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Dave Chinner <david@fromorbit.com>,
        Chuck Lever <chuck.lever@oracle.com>, Jan Kara <jack@suse.cz>,
        Amir Goldstein <amir73il@gmail.com>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-xfs@vger.kernel.org, linux-mm@kvack.org,
        linux-nfs@vger.kernel.org
Date:   Wed, 26 Apr 2023 05:46:25 -0400
In-Reply-To: <20230426-meerblick-tortur-c6606f6126fa@brauner>
References: <20230424151104.175456-1-jlayton@kernel.org>
         <20230424151104.175456-2-jlayton@kernel.org>
         <20230426-meerblick-tortur-c6606f6126fa@brauner>
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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 th=
e
> > 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.
> >=20
> > 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).
> >=20
> > 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.
> >=20
> > What we need is a way to only use fine-grained timestamps when they are
> > being actively queried:
> >=20
> > 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.
> >=20
> > 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 filesyste=
ms
> > to opt-in to this behavior.
> >=20
> > It then adds a new current_ctime function that acts like the
> > current_time helper, but will conditionally grab fine-grained timestamp=
s
> > 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 inod=
e
> > and atomically marking the ctime as queried.
> >=20
> > Later patches will convert filesystems over to this new scheme.
> >=20
> > 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(-)
> >=20
> > 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 timespe=
c64 *now)
> >  {
> >  	int sync_it =3D 0;
> > +	struct timespec64 ctime =3D inode->i_ctime;
> > =20
> >  	/* 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 =3D S_MTIME;
> > =20
> > -	if (!timespec64_equal(&inode->i_ctime, now))
> > +	if (is_multigrain_ts(inode))
> > +		ctime.tv_nsec &=3D ~I_CTIME_QUERIED;
> > +	if (!timespec64_equal(&ctime, now))
> >  		sync_it |=3D S_CTIME;
> > =20
> >  	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;
> >  }
> > =20
> > +/**
> > + * 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 b=
een
> > + * QUERIED, then get a fine-grained timestamp.
> > + */
> > +struct timespec64 current_ctime(struct inode *inode)
> > +{
> > +	struct timespec64 now;
> > +	long nsec =3D 0;
> > +	bool multigrain =3D is_multigrain_ts(inode);
> > +
> > +	if (multigrain) {
> > +		atomic_long_t *pnsec =3D (atomic_long_t *)&inode->i_ctime.tv_nsec;
> > +
> > +		nsec =3D 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 =3D inode->i_ctime;
> > +
> > +			if (timespec64_compare(&ctime, &now) > 0)
> > +				now =3D 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 =3D file_inode(file);
> > -	struct timespec64 now =3D current_time(inode);
> > +	struct timespec64 now =3D current_ctime(inode);
> > =20
> >  	ret =3D inode_needs_update_time(inode, &now);
> >  	if (ret <=3D 0)
> > @@ -2109,7 +2156,7 @@ static int file_modified_flags(struct file *file,=
 int flags)
> >  {
> >  	int ret;
> >  	struct inode *inode =3D file_inode(file);
> > -	struct timespec64 now =3D current_time(inode);
> > +	struct timespec64 now =3D current_ctime(inode);
> > =20
> >  	/*
> >  	 * Clear the security bits if the process is not being run by root.
> > @@ -2419,9 +2466,11 @@ struct timespec64 timestamp_truncate(struct time=
spec64 t, struct inode *inode)
> >  	if (unlikely(t.tv_sec =3D=3D sb->s_time_max || t.tv_sec =3D=3D sb->s_=
time_min))
> >  		t.tv_nsec =3D 0;
> > =20
> > -	/* 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 =3D=3D 1)
> >  		; /* nothing */
> > +	else if (gran =3D=3D 2)
> > +		t.tv_nsec &=3D ~1L;
>=20
> 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.
--=20
Jeff Layton <jlayton@kernel.org>