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

Jan Kara
2023-05-23T10:17:30Z

On Tue 23-05-23 12:02:40, 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@kernel.org>
>
> 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.

Also as I'm thinking about it your current scheme is slightly racy. Suppose
the filesystem does:

CPU1 CPU2

statx()
inode->i_ctime = current_time()
current_mg_time()
nsec = atomic_long_fetch_andnot(QUERIED, &inode->i_ctime.tv_nsec)
nsec = atomic_long_fetch_or(QUERIED, &inode->i_ctime.tv_nsec)
if (nsec & QUERIED) - not set
ktime_get_coarse_real_ts64(&now)
return timestamp_truncate(now, inode);
- QUERIED flag in the inode->i_ctime gets overwritten by the assignment
=> we need not update ctime due to granularity although it was queried

One more reason to use explicit function to update inode->i_ctime ;)

Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR