Re: [PATCH] nfsd: special case truncates some more

[Christoph Hellwig <hch@lst.de>]

On Mon, Jan 23, 2017 at 10:52:09AM -0500, Jeff Layton wrote:
> To be clear, the client is requesting to set the mtime to current server
> time and not to a specific mtime, right?

Yes.  And I think it's mostly the Linux client being lazy - ATTR_MTIME
is what it gets from the VFS for a truncate operation (but not ftrunate,
so we probably won't see it on the wire in that case, but I need to verify
that first).  Yet another reason for ->truncate :)

> I don't see where vfs_truncate will handle the times though. do_truncate
> will, but you have to pass in a non-zero time_attrs and vfs_truncate
> always sets that to 0.

This is the magic of the Linux VFS interface.  For a ATTR_SIZE operation
the file system is expected to update mtime and ctime if the size changes
even if ATTR_MTIME and ATTR_CTIME are not set.  See the comments
in xfs_vn_setattr_size, which I wrote many years ago when I tripped
over this interesting calling convention.

> If we did want to do this, it seems like it might be better to just add
> a new time_attrs arg to vfs_truncate that gets passed to do_truncate.
> Most callers would set it to zero, but nfsd could set it to:
> 
>     iap->ia_valid & (ATTR_MTIME|ATTR_CTIME)
> 
> Would that work?

I'd hate it.  I'd rather spent my time on a real truncate operation
which makes all the above magic explicit, and as a side effect would
fix the Linux client sending spurious mtime update requests that
the procotol already requires to be done implicitly.