Re: [PATCH v7 12/13] ext4: switch to multigrain timestamps

Jan Kara
2023-09-20T15:45:33Z

On Wed 20-09-23 10:12:03, Jeff Layton wrote:
> On Wed, 2023-09-20 at 14:48 +0200, Jan Kara wrote:
> > On Wed 20-09-23 06:35:18, Jeff Layton wrote:
> > > On Wed, 2023-09-20 at 12:17 +0200, Jan Kara wrote:
> > > > If I were a sysadmin, I'd rather opt for something like
> > > > finegrained timestamps + lazytime (if I needed the finegrained timestamps
> > > > functionality). That should avoid the IO overhead of finegrained timestamps
> > > > as well and I'd know I can have problems with timestamps only after a
> > > > system crash.
> > >
> > > > I've just got another idea how we could solve the problem: Couldn't we
> > > > always just report coarsegrained timestamp to userspace and provide access
> > > > to finegrained value only to NFS which should know what it's doing?
> > > >
> > >
> > > I think that'd be hard. First of all, where would we store the second
> > > timestamp? We can't just truncate the fine-grained ones to come up with
> > > a coarse-grained one. It might also be confusing having nfsd and local
> > > filesystems present different attributes.
> >
> > So what I had in mind (and I definitely miss all the NFS intricacies so the
> > idea may be bogus) was that inode->i_ctime would be maintained exactly as
> > is now. There will be new (kernel internal at least for now) STATX flag
> > STATX_MULTIGRAIN_TS. fill_mg_cmtime() will return timestamp truncated to
> > sb->s_time_gran unless STATX_MULTIGRAIN_TS is set. Hence unless you set
> > STATX_MULTIGRAIN_TS, there is no difference in the returned timestamps
> > compared to the state before multigrain timestamps were introduced. With
> > STATX_MULTIGRAIN_TS we return full precision timestamp as stored in the
> > inode. Then NFS in fh_fill_pre_attrs() and fh_fill_post_attrs() needs to
> > make sure STATX_MULTIGRAIN_TS is set when calling vfs_getattr() to get
> > multigrain time.
>
> > I agree nfsd may now be presenting slightly different timestamps than user
> > is able to see with stat(2) directly on the filesystem. But is that a
> > problem? Essentially it is a similar solution as the mgtime mount option
> > but now sysadmin doesn't have to decide on filesystem mount how to report
> > timestamps but the stat caller knowingly opts into possibly inconsistent
> > (among files) but high precision timestamps. And in the particular NFS
> > usecase where stat is called all the time anyway, timestamps will likely
> > even be consistent among files.
> >
>
> I like this idea...
>
> Would we also need to raise sb->s_time_gran to something corresponding
> to HZ on these filesystems?

I was actually confused a bit about how timestamp_truncate() works. The
jiffie granularity is just direct consequence of current_time() using
ktime_get_coarse_real_ts64() and not of timestamp_truncate().
sb->s_time_gran seems to be more about the on-disk format so it doesn't
seem like a great idea to touch it. So probably we can just truncate
timestamps in generic_fillattr() to HZ granularity unconditionally.

> If we truncate the timestamps at a granularity corresponding to HZ before
> presenting them via statx and the like then that should work around the
> problem with programs that compare timestamps between inodes.

Exactly.

> With NFSv4, when a filesystem doesn't report a STATX_CHANGE_COOKIE, nfsd
> will fake one up using the ctime. It's fine for that to use a full fine-
> grained timestamp since we don't expect to be able to compare that value
> with one of a different inode.

Yes.

> I think we'd want nfsd to present the mtime/ctime values as truncated,
> just like we would with a local fs. We could hit the same problem of an
> earlier-looking timestamp with NFS if we try to present the actual fine-
> grained values to the clients. IOW, I'm convinced that we need to avoid
> this behavior in most situations.

I wasn't sure if there's a way to do this within NFS - i.e., if the value
communicated via NFSv3 protocol (I know v4 has a special change cookie
field for it) that gets used for detecting need to revalidate file contents
isn't the one presented to client's userspace as ctime. If there's a way to
do this then great, I'm all for presenting truncated timestamps even for
NFS.

> If we do this, then we technically don't need the mount option either.

Yes, that was my hope.

> We could still add it though, and have it govern whether fill_mg_cmtime
> truncates the timestamps before storing them in the kstat.

Well, if we decide these timestamps are useful for userspace as well, I'd
rather make that a userspace visible STATX flag than a mount option. So
applications aware of the pitfalls can get high precision timestamps
without possibly breaking unaware applications.

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