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

[Christian Brauner]

On Wed, Aug 02, 2023 at 04:54:09PM -0400, Jeff Layton wrote:
> On Wed, 2023-08-02 at 21:35 +0200, Jan Kara wrote:
> > 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@xxxxxxxxxx>
> > > ---
> > >  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...
> > 
> 
> Technically, they already devolve to current_coarse_time anyway, but
> changing them would allow them to skip the fstype flag check, but I like
> your idea below better anyway.
> 
> > > @@ -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
> > 
> 
> This is a great idea. I'll rework the series along the lines you
> suggest. That also answers my earlier question to Christian:
> 
> I'll just resend the whole series (it's not very big anyway), and I'll
> include the fill_mg_cmtime prototype change.

Ok, sound good!