Re: [PATCH v5 6/9] xfs: switch to multigrain timestamps

Darrick J. Wong
2024-07-11T19:14:35Z

On Thu, Jul 11, 2024 at 11:58:59AM -0400, Jeff Layton wrote:
> On Thu, 2024-07-11 at 08:09 -0700, Darrick J. Wong wrote:
> > On Thu, Jul 11, 2024 at 07:08:10AM -0400, Jeff Layton wrote:
> > > Enable multigrain timestamps, which should ensure that there is an
> > > apparent change to the timestamp whenever it has been written after
> > > being actively observed via getattr.
> > >
> > > Also, anytime the mtime changes, the ctime must also change, and those
> > > are now the only two options for xfs_trans_ichgtime. Have that function
> > > unconditionally bump the ctime, and ASSERT that XFS_ICHGTIME_CHG is
> > > always set.
> > >
> > > Finally, stop setting STATX_CHANGE_COOKIE in getattr, since the ctime
> > > should give us better semantics now.
> >
> > Following up on "As long as the fs isn't touching i_ctime_nsec directly,
> > you shouldn't need to worry about this" from:
> > https://lore.kernel.org/linux-xfs/cae5c28f172ac57b7eaaa98a00b23f342f01ba64.camel@kernel.org/
> >
> > xfs /does/ touch i_ctime_nsec directly when it's writing inodes to disk.
> > From xfs_inode_to_disk, see:
> >
> > to->di_ctime = xfs_inode_to_disk_ts(ip, inode_get_ctime(inode));
> >
> > AFAICT, inode_get_ctime itself remains unchanged, and still returns
> > inode->__i_ctime, right?  In which case it's returning a raw timespec64,
> > which can include the QUERIED flag in tv_nsec, right?
> >
>
> No, in the first patch in the series, inode_get_ctime becomes this:
>
> #define I_CTIME_QUERIED ((u32)BIT(31))
>
> static inline time64_t inode_get_ctime_sec(const struct inode *inode)
> {
> return inode->i_ctime_sec;
> }
>
> static inline long inode_get_ctime_nsec(const struct inode *inode)
> {
> return inode->i_ctime_nsec & ~I_CTIME_QUERIED;
> }
>
> static inline struct timespec64 inode_get_ctime(const struct inode *inode)
> {
> struct timespec64 ts = { .tv_sec = inode_get_ctime_sec(inode),
> .tv_nsec = inode_get_ctime_nsec(inode) };
>
> return ts;
> }

Doh! I forgot that this has already been soaking in the vfs tree:
https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/include/linux/fs.h?h=next-20240711&id=3aa63a569c64e708df547a8913c84e64a06e7853

> ...which should ensure that you never store the QUERIED bit.

So yep, we're fine here. Sorry about the noise; this was the very
subtle clue in the diff that the change had already been applied:

static inline struct timespec64 inode_get_ctime(const struct inode *inode)
@@ -1626,13 +1637,7 @@ static inline struct timespec64 inode_get_ctime(const struct inode *inode)
return ts;
}

(Doh doh doh doh doh...)

> > Now let's look at the consumer:
> >
> > static inline xfs_timestamp_t
> > xfs_inode_to_disk_ts(
> > struct xfs_inode *ip,
> > const struct timespec64 tv)
> > {
> > struct xfs_legacy_timestamp *lts;
> > xfs_timestamp_t ts;
> >
> > if (xfs_inode_has_bigtime(ip))
> > return cpu_to_be64(xfs_inode_encode_bigtime(tv));
> >
> > lts = (struct xfs_legacy_timestamp *)&ts;
> > lts->t_sec = cpu_to_be32(tv.tv_sec);
> > lts->t_nsec = cpu_to_be32(tv.tv_nsec);
> >
> > return ts;
> > }
> >
> > For the !bigtime case (aka before we added y2038 support) the queried
> > flag gets encoded into the tv_nsec field since xfs doesn't filter the
> > queried flag.
> >
> > For the bigtime case, the timespec is turned into an absolute nsec count
> > since the xfs epoch (which is the minimum timestamp possible under the
> > old encoding scheme):
> >
> > static inline uint64_t xfs_inode_encode_bigtime(struct timespec64 tv)
> > {
> > return xfs_unix_to_bigtime(tv.tv_sec) * NSEC_PER_SEC + tv.tv_nsec;
> > }
> >
> > Here we'd also be mixing in the QUERIED flag, only now we've encoded a
> > time that's a second in the future.  I think the solution is to add a:
> >
> > static inline struct timespec64
> > inode_peek_ctime(const struct inode *inode)
> > {
> > return (struct timespec64){
> > .tv_sec = inode->__i_ctime.tv_sec,
> > .tv_nsec = inode->__i_ctime.tv_nsec & ~I_CTIME_QUERIED,
> > };
> > }
> >
> > similar to what inode_peek_iversion does for iversion; and then
> > xfs_inode_to_disk can do:
> >
> > to->di_ctime = xfs_inode_to_disk_ts(ip, inode_peek_ctime(inode));
> >
> > which would prevent I_CTIME_QUERIED from going out to disk.
> >
> > At load time, xfs_inode_from_disk uses inode_set_ctime_to_ts so I think
> > xfs won't accidentally introduce QUERIED when it's loading an inode from
> > disk.
> >
> >
>
> Also already done in this patchset:
>
> struct timespec64 inode_set_ctime_to_ts(struct inode *inode, struct timespec64 ts)
> {
> inode->i_ctime_sec = ts.tv_sec;
> inode->i_ctime_nsec = ts.tv_nsec & ~I_CTIME_QUERIED;
> trace_inode_set_ctime_to_ts(inode, &ts);
> return ts;
> }
> EXPORT_SYMBOL(inode_set_ctime_to_ts);
>
> Basically, we never want to store or fetch the QUERIED flag from disk,
> and since it's in an unused bit, we can just universally mask it off
> when dealing with "external" users of it.
>
> One caveat -- I am using the sign bit for the QUERIED flag, so I'm
> assuming that no one should ever pass inode_set_ctime_to_ts a negative
> tv_nsec value.
>
> Maybe I should add a WARN_ON_ONCE here to check for that? It seems
> nonsensical, but you never know...

Well in theory filesystems should validate incoming timestamps and
reject tv_nsec with the high bit set, but I'd bet there's a filesystem
out there that allows negative nanoseconds, even if the kernel will
never pass it such a thing. ;)

> > > Signed-off-by: Jeff Layton <jlayton@kernel.org>
> > > ---
> > >  fs/xfs/libxfs/xfs_trans_inode.c |  6 +++---
> > >  fs/xfs/xfs_iops.c               | 10 +++-------
> > >  fs/xfs/xfs_super.c              |  2 +-
> > >  3 files changed, 7 insertions(+), 11 deletions(-)
> > >
> > > diff --git a/fs/xfs/libxfs/xfs_trans_inode.c b/fs/xfs/libxfs/xfs_trans_inode.c
> > > index 69fc5b981352..1f3639bbf5f0 100644
> > > --- a/fs/xfs/libxfs/xfs_trans_inode.c
> > > +++ b/fs/xfs/libxfs/xfs_trans_inode.c
> > > @@ -62,12 +62,12 @@ xfs_trans_ichgtime(
> > >   ASSERT(tp);
> > >   xfs_assert_ilocked(ip, XFS_ILOCK_EXCL);
> > >
> > > - tv = current_time(inode);
> > > + /* If the mtime changes, then ctime must also change */
> > > + ASSERT(flags & XFS_ICHGTIME_CHG);
> > >
> > > + tv = inode_set_ctime_current(inode);
> > >   if (flags & XFS_ICHGTIME_MOD)
> > >   inode_set_mtime_to_ts(inode, tv);
> > > - if (flags & XFS_ICHGTIME_CHG)
> > > - inode_set_ctime_to_ts(inode, tv);
> > >   if (flags & XFS_ICHGTIME_CREATE)
> > >   ip->i_crtime = tv;

And as I mentioned elsewhere in this thread, 6.11 contains a change to
make it so that xfs_trans_ichgtime can set the access time. That breaks
the old assertion that XFS_ICHGTIME_CHG is always set, but I think we
can work around that easily.

if (flags & XFS_ICHGTIME_CHG)
tv = inode_set_ctime_current(inode);
else
tv = current_time(inode);

--D

> > >  }
> > > diff --git a/fs/xfs/xfs_iops.c b/fs/xfs/xfs_iops.c
> > > index a00dcbc77e12..d25872f818fa 100644
> > > --- a/fs/xfs/xfs_iops.c
> > > +++ b/fs/xfs/xfs_iops.c
> > > @@ -592,8 +592,9 @@ xfs_vn_getattr(
> > >   stat->gid = vfsgid_into_kgid(vfsgid);
> > >   stat->ino = ip->i_ino;
> > >   stat->atime = inode_get_atime(inode);
> > > - stat->mtime = inode_get_mtime(inode);
> > > - stat->ctime = inode_get_ctime(inode);
> > > +
> > > + fill_mg_cmtime(stat, request_mask, inode);
> > > +
> > >   stat->blocks = XFS_FSB_TO_BB(mp, ip->i_nblocks + ip->i_delayed_blks);
> > >
> > >   if (xfs_has_v3inodes(mp)) {
> > > @@ -603,11 +604,6 @@ xfs_vn_getattr(
> > >   }
> > >   }
> > >
> > > - if ((request_mask & STATX_CHANGE_COOKIE) && IS_I_VERSION(inode)) {
> > > - stat->change_cookie = inode_query_iversion(inode);
> > > - stat->result_mask |= STATX_CHANGE_COOKIE;
> > > - }
> > > -
> > >   /*
> > >   * Note: If you add another clause to set an attribute flag, please
> > >   * update attributes_mask below.
> > > diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
> > > index 27e9f749c4c7..210481b03fdb 100644
> > > --- a/fs/xfs/xfs_super.c
> > > +++ b/fs/xfs/xfs_super.c
> > > @@ -2052,7 +2052,7 @@ static struct file_system_type xfs_fs_type = {
> > >   .init_fs_context = xfs_init_fs_context,
> > >   .parameters = xfs_fs_parameters,
> > >   .kill_sb = xfs_kill_sb,
> > > - .fs_flags = FS_REQUIRES_DEV | FS_ALLOW_IDMAP,
> > > + .fs_flags = FS_REQUIRES_DEV | FS_ALLOW_IDMAP | FS_MGTIME,
> > >  };
> > >  MODULE_ALIAS_FS("xfs");
> > >
> > >
> > > --
> > > 2.45.2
> > >
>
> --
> Jeff Layton <jlayton@kernel.org>