Re: [PATCH v4 2/9] fs: add infrastructure for multigrain inode i_m/ctime

[Christian Brauner]

On Tue, May 23, 2023 at 12:02:40PM +0200, Jan Kara wrote:
> On Thu 18-05-23 07:47:35, 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 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. 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, 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.
> > 
> > The kernel always stores normalized ctime values, so only the first 30
> > bits of the tv_nsec field are ever used. Whenever the mtime changes, the
> > ctime must also change.
> > 
> > Use the 31st bit of the ctime tv_nsec field to indicate that something
> > has queried the inode for the i_mtime or i_ctime. When this flag is set,
> > on the next timestamp update, the kernel can fetch a fine-grained
> > timestamp instead of the usual coarse-grained one.
> > 
> > This patch adds the infrastructure this scheme. Filesytems can opt
> > into it by setting the FS_MULTIGRAIN_TS flag in the fstype.
> > 
> > Later patches will convert individual filesystems over to use it.
> > 
> > Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
> 
> So there are two things I dislike about this series because I think they
> are fragile:
> 
> 1) If we have a filesystem supporting multigrain ts and someone
> accidentally directly uses the value of inode->i_ctime, he can get bogus
> value (with QUERIED flag). This mistake is very easy to do. So I think we
> should rename i_ctime to something like __i_ctime and always use accessor
> function for it.
> 
> 2) As I already commented in a previous version of the series, the scheme
> with just one flag for both ctime and mtime and flag getting cleared in
> current_time() relies on the fact that filesystems always do an equivalent
> of:
> 
> 	inode->i_mtime = inode->i_ctime = current_time();
> 
> Otherwise we can do coarse grained update where we should have done a fine
> grained one. Filesystems often update timestamps like this but not
> universally. Grepping shows some instances where only inode->i_mtime is set
> from current_time() e.g. in autofs or bfs. Again a mistake that is rather
> easy to make and results in subtle issues. I think this would be also
> nicely solved by renaming i_ctime to __i_ctime and using a function to set
> ctime. Mtime could then be updated with inode->i_mtime = ctime_peek().
> 
> I understand this is quite some churn but a very mechanical one that could
> be just done with Coccinelle and a few manual fixups. So IMHO it is worth
> the more robust result.

Yeah, these are all good points.

> 
> Some more nits below.
> 
> > +/**
> > + * current_mg_time - 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.
> > + */
> 
> The comment should also mention that QUERIED flag is cleared from the ctime.
> 
> > +static struct timespec64 current_mg_time(struct inode *inode)
> > +{
> > +	struct timespec64 now;
> > +	atomic_long_t *pnsec = (atomic_long_t *)&inode->i_ctime.tv_nsec;
> > +	long nsec = atomic_long_fetch_andnot(I_CTIME_QUERIED, 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 = ctime_peek(inode);
> > +		if (timespec64_compare(&ctime, &now) > 0)
> > +			now = ctime;
> > +	}
> > +
> > +	return now;
> > +}
> > +
> 
> ...
> 
> > +/**
> > + * ctime_nsec_peek - peek at (but don't query) the ctime tv_nsec field
> > + * @inode: inode to fetch the ctime from
> > + *
> > + * 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 long ctime_nsec_peek(const struct inode *inode)
> > +{
> > +	return inode->i_ctime.tv_nsec &~ I_CTIME_QUERIED;
> 
> This is somewhat unusual spacing. I'd use:
> 
> 	inode->i_ctime.tv_nsec & ~I_CTIME_QUERIED
> 
> > +}
> > +
> > +/**
> > + * ctime_peek - peek at (but don't query) the ctime
> > + * @inode: inode to fetch the ctime from
> > + *
> > + * Grab the current ctime from the inode, sans I_CTIME_QUERIED flag. For
> > + * use by internal consumers that don't require a fine-grained update on
> > + * the next change.
> > + *
> > + * This is safe to call regardless of whether the underlying filesystem
> > + * is using multigrain timestamps.
> > + */
> > +static inline struct timespec64 ctime_peek(const struct inode *inode)
> > +{
> > +	struct timespec64 ctime;
> > +
> > +	ctime.tv_sec = inode->i_ctime.tv_sec;
> > +	ctime.tv_nsec = ctime_nsec_peek(inode);
> > +
> > +	return ctime;
> > +}
> 
> Given this is in a header that gets included in a lot of places, maybe we
> should call it like inode_ctime_peek() or inode_ctime_get() to reduce
> chances of a name clash?

I think I mentioned this in an earlier comment. Independent of this
series, it would be kinda nice if we could start moving stuff out of
fs.h so we end up with a finer grained split of fs.h.