Re: [PATCH v2 5/7] fs: add an ATTR_CTIME_DLG flag

[Jeff Layton <jlayton@kernel.org>]

On Mon, 2024-08-26 at 15:08 +0200, Christian Brauner wrote:
> On Mon, Aug 26, 2024 at 08:46:15AM GMT, 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 does send a mtime value. In the case where
> > it does, then we may also need to update the ctime to a value
> > commensurate with that.
> > 
> > Add a ATTR_CTIME_DELEG flag, which tells the underlying setattr
> 
> Fwiw: disconnect between commit message and actually used ATTR_CTIME_DLG.
> 

Thanks, will fix.

> > machinery to respect that value and not to set it to the current time.
> > 
> > Signed-off-by: Jeff Layton <jlayton@kernel.org>
> > ---
> 
> Are you set on sending us on a mission to free up ATTR_* bits after
> freeing up FMODE_* bits? ;)
> 

Those aren't usually allocated from the heap, so I wouldn't bother, but
I get the jest.

> If there's going to be more ATTR_*DELEG* flags that modify the
> behavior when delegation is in effect then we could consider adding
> another unsigned int ia_deleg field to struct iattr so that you can check:
> 
> if (ia_valid & ATTR_CTIME) {
> 	if (unlikely(iattr->ia_deleg & ATTR_CTIME))
> 		// do some special stuff
> 	else
> 		// do the regular stuff
> }
> 
> or some such variant.
> 

I don't forsee other flags being needed, but you never know. For now I
wouldn't bother.

> >  fs/attr.c          | 10 +++++++++-
> >  include/linux/fs.h |  1 +
> >  2 files changed, 10 insertions(+), 1 deletion(-)
> > 
> > diff --git a/fs/attr.c b/fs/attr.c
> > index 7144b207e715..0eb7b228b94d 100644
> > --- a/fs/attr.c
> > +++ b/fs/attr.c
> > @@ -295,7 +295,15 @@ static void setattr_copy_mgtime(struct inode *inode, const struct iattr *attr)
> >  		return;
> >  	}
> >  
> > -	now = inode_set_ctime_current(inode);
> > +	/*
> > +	 * 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_CTIME_DLG)
> > +		inode_set_ctime_to_ts(inode, attr->ia_ctime);
> > +	else
> > +		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)
> > diff --git a/include/linux/fs.h b/include/linux/fs.h
> > index 7c1da3c687bd..43a802b2cb0d 100644
> > --- a/include/linux/fs.h
> > +++ b/include/linux/fs.h
> > @@ -211,6 +211,7 @@ typedef int (dio_iodone_t)(struct kiocb *iocb, loff_t offset,
> >  #define ATTR_TIMES_SET	(1 << 16)
> >  #define ATTR_TOUCH	(1 << 17)
> >  #define ATTR_DELEG	(1 << 18) /* Delegated attrs (don't break) */
> > +#define ATTR_CTIME_DLG	(1 << 19) /* Delegation in effect */
> 
> What's the interaction between ATTR_DELEG and ATTR_CTIME_DLG? I think
> that's potentially confusing.
> 

Now that you mention it, I suppose we could just key off of ATTR_DELEG
instead of declaring a new flag. That should be simpler to reason out
for everyone. I'll respin this along those lines instead.

Thanks for the review!
-- 
Jeff Layton <jlayton@kernel.org>