Re: [PATCH v4 09/11] fs: handle delegated timestamps in setattr_copy_mgtime

[Jeff Layton <jlayton@kernel.org>]

On Thu, 2024-09-05 at 11:44 -0400, Chuck Lever wrote:
> On Thu, Sep 05, 2024 at 08:41:53AM -0400, Jeff Layton wrote:
> > When updating the ctime on an inode for a SETATTR with a multigrain
> > filesystem, we usually want to take the latest time we can get for the
> > ctime. The exception to this rule is when there is a nfsd write
> > delegation and the server is proxying timestamps from the client.
> > 
> > When nfsd gets a CB_GETATTR response, we want to update the timestamp
> > value in the inode to the values that the client is tracking. The client
> > doesn't send a ctime value (since that's always determined by the
> > exported filesystem), but it can send a mtime value. In the case where
> > it does, then we may need to update the ctime to a value commensurate
> > with that instead of the current time.
> > 
> > If ATTR_DELEG is set, then use ia_ctime value instead of setting the
> > timestamp to the current time.
> > 
> > With the addition of delegated timestamps we can also receive a request
> > to update only the atime, but we may not need to set the ctime. Trust
> > the ATTR_CTIME flag in the update and only update the ctime when it's
> > set.
> > 
> > Signed-off-by: Jeff Layton <jlayton@kernel.org>
> > ---
> >  fs/attr.c          | 28 +++++++++++++--------
> >  fs/inode.c         | 74 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
> >  include/linux/fs.h |  2 ++
> >  3 files changed, 94 insertions(+), 10 deletions(-)
> > 
> > diff --git a/fs/attr.c b/fs/attr.c
> > index 3bcbc45708a3..392eb62aa609 100644
> > --- a/fs/attr.c
> > +++ b/fs/attr.c
> > @@ -286,16 +286,20 @@ static void setattr_copy_mgtime(struct inode *inode, const struct iattr *attr)
> >  	unsigned int ia_valid = attr->ia_valid;
> >  	struct timespec64 now;
> >  
> > -	/*
> > -	 * If the ctime isn't being updated then nothing else should be
> > -	 * either.
> > -	 */
> > -	if (!(ia_valid & ATTR_CTIME)) {
> > -		WARN_ON_ONCE(ia_valid & (ATTR_ATIME|ATTR_MTIME));
> > -		return;
> > +	if (ia_valid & ATTR_CTIME) {
> > +		/*
> > +		 * In the case of an update for a write delegation, we must respect
> > +		 * the value in ia_ctime and not use the current time.
> > +		 */
> > +		if (ia_valid & ATTR_DELEG)
> > +			now = inode_set_ctime_deleg(inode, attr->ia_ctime);
> > +		else
> > +			now = inode_set_ctime_current(inode);
> > +	} else {
> > +		/* If ATTR_CTIME isn't set, then ATTR_MTIME shouldn't be either. */
> > +		WARN_ON_ONCE(ia_valid & ATTR_MTIME);
> >  	}
> >  
> > -	now = inode_set_ctime_current(inode);
> >  	if (ia_valid & ATTR_ATIME_SET)
> >  		inode_set_atime_to_ts(inode, attr->ia_atime);
> >  	else if (ia_valid & ATTR_ATIME)
> > @@ -354,8 +358,12 @@ void setattr_copy(struct mnt_idmap *idmap, struct inode *inode,
> >  		inode_set_atime_to_ts(inode, attr->ia_atime);
> >  	if (ia_valid & ATTR_MTIME)
> >  		inode_set_mtime_to_ts(inode, attr->ia_mtime);
> > -	if (ia_valid & ATTR_CTIME)
> > -		inode_set_ctime_to_ts(inode, attr->ia_ctime);
> > +	if (ia_valid & ATTR_CTIME) {
> > +		if (ia_valid & ATTR_DELEG)
> > +			inode_set_ctime_deleg(inode, attr->ia_ctime);
> > +		else
> > +			inode_set_ctime_to_ts(inode, attr->ia_ctime);
> > +	}
> >  }
> >  EXPORT_SYMBOL(setattr_copy);
> >  
> 
> This patch fails to apply cleanly to my copy of nfsd-next:
> 
>   error: `git apply --index`: error: patch failed: fs/attr.c:286
>   error: fs/attr.c: patch does not apply
> 
> Before I try jiggling this to get it to apply, is there anything
> I should know? I worry about a potential merge conflict here,
> hopefully it will be no more complicated than that.
> 
> 

This is based on a combo of your nfsd-next branch and Christian's
vfs.mgtime branch. This patch in particular depends on the multigrain
patches in his tree.

I think we can just drop this patch from the series in your tree, and
I'll get Christian to pick it up in his tree. The ctime setting will be
a bit racy without it but everything should still work.

Sound ok? Christian, are you OK with picking this up in vfs.mgtime?

> > diff --git a/fs/inode.c b/fs/inode.c
> > index 01f7df1973bd..f0fbfd470d8e 100644
> > --- a/fs/inode.c
> > +++ b/fs/inode.c
> > @@ -2835,6 +2835,80 @@ struct timespec64 inode_set_ctime_current(struct inode *inode)
> >  }
> >  EXPORT_SYMBOL(inode_set_ctime_current);
> >  
> > +/**
> > + * inode_set_ctime_deleg - try to update the ctime on a delegated inode
> > + * @inode: inode to update
> > + * @update: timespec64 to set the ctime
> > + *
> > + * Attempt to atomically update the ctime on behalf of a delegation holder.
> > + *
> > + * The nfs server can call back the holder of a delegation to get updated
> > + * inode attributes, including the mtime. When updating the mtime we may
> > + * need to update the ctime to a value at least equal to that.
> > + *
> > + * This can race with concurrent updates to the inode, in which
> > + * case we just don't do the update.
> > + *
> > + * Note that this works even when multigrain timestamps are not enabled,
> > + * so use it in either case.
> > + */
> > +struct timespec64 inode_set_ctime_deleg(struct inode *inode, struct timespec64 update)
> > +{
> > +	ktime_t now, floor = atomic64_read(&ctime_floor);
> > +	struct timespec64 now_ts, cur_ts;
> > +	u32 cur, old;
> > +
> > +	/* pairs with try_cmpxchg below */
> > +	cur = smp_load_acquire(&inode->i_ctime_nsec);
> > +	cur_ts.tv_nsec = cur & ~I_CTIME_QUERIED;
> > +	cur_ts.tv_sec = inode->i_ctime_sec;
> > +
> > +	/* If the update is older than the existing value, skip it. */
> > +	if (timespec64_compare(&update, &cur_ts) <= 0)
> > +		return cur_ts;
> > +
> > +	now = coarse_ctime(floor);
> > +	now_ts = ktime_to_timespec64(now);
> > +
> > +	/* Clamp the update to "now" if it's in the future */
> > +	if (timespec64_compare(&update, &now_ts) > 0)
> > +		update = now_ts;
> > +
> > +	update = timestamp_truncate(update, inode);
> > +
> > +	/* No need to update if the values are already the same */
> > +	if (timespec64_equal(&update, &cur_ts))
> > +		return cur_ts;
> > +
> > +	/*
> > +	 * Try to swap the nsec value into place. If it fails, that means
> > +	 * we raced with an update due to a write or similar activity. That
> > +	 * stamp takes precedence, so just skip the update.
> > +	 */
> > +retry:
> > +	old = cur;
> > +	if (try_cmpxchg(&inode->i_ctime_nsec, &cur, update.tv_nsec)) {
> > +		inode->i_ctime_sec = update.tv_sec;
> > +		mgtime_counter_inc(mg_ctime_swaps);
> > +		return update;
> > +	}
> > +
> > +	/*
> > +	 * Was the change due to someone marking the old ctime QUERIED?
> > +	 * If so then retry the swap. This can only happen once since
> > +	 * the only way to clear I_CTIME_QUERIED is to stamp the inode
> > +	 * with a new ctime.
> > +	 */
> > +	if (!(old & I_CTIME_QUERIED) && (cur == (old | I_CTIME_QUERIED)))
> > +		goto retry;
> > +
> > +	/* Otherwise, it was a new timestamp. */
> > +	cur_ts.tv_sec = inode->i_ctime_sec;
> > +	cur_ts.tv_nsec = cur & ~I_CTIME_QUERIED;
> > +	return cur_ts;
> > +}
> > +EXPORT_SYMBOL(inode_set_ctime_deleg);
> > +
> >  /**
> >   * in_group_or_capable - check whether caller is CAP_FSETID privileged
> >   * @idmap:	idmap of the mount @inode was found from
> > diff --git a/include/linux/fs.h b/include/linux/fs.h
> > index eff688e75f2f..ea7ed437d2b1 100644
> > --- a/include/linux/fs.h
> > +++ b/include/linux/fs.h
> > @@ -1544,6 +1544,8 @@ static inline bool fsuidgid_has_mapping(struct super_block *sb,
> >  
> >  struct timespec64 current_time(struct inode *inode);
> >  struct timespec64 inode_set_ctime_current(struct inode *inode);
> > +struct timespec64 inode_set_ctime_deleg(struct inode *inode,
> > +					struct timespec64 update);
> >  
> >  static inline time64_t inode_get_atime_sec(const struct inode *inode)
> >  {
> > 
> > -- 
> > 2.46.0
> > 
> 

-- 
Jeff Layton <jlayton@kernel.org>