Re: [PATCH against 2.6.14] truncate() or ftruncate shouldn't change mtime if size doesn't change.

[Neil Brown <neilb@suse.de>]

On Monday October 31, aia21@cam.ac.uk wrote:
> > 
> > This partially obsoletes the similar optimisation in inode_setattr().  I
> > guess the optimisation there retains some usefulness for O_TRUNC opens of
> > zero-length files, but for symettry and micro-efficiency, perhaps we should
> > remvoe the inode_setattr() test and check for i_size==0 in may_open()?
> 
> Sounds like a good idea.  That does simplify inode_setattr() nicely...
> 
> Signed-off-by: Anton Altaparmakov <aia21@cantab.net>
> 
> --- linux-2.6/fs/attr.c.old	2005-10-31 09:29:38.000000000 +0000
> +++ linux-2.6/fs/attr.c	2005-10-31 09:30:39.000000000 +0000
> @@ -70,19 +70,10 @@ int inode_setattr(struct inode * inode, 
>  	int error = 0;
>  
>  	if (ia_valid & ATTR_SIZE) {
> -		if (attr->ia_size != i_size_read(inode)) {
> -			error = vmtruncate(inode, attr->ia_size);
> -			if (error || (ia_valid == ATTR_SIZE))
> -				goto out;
> -		} else {
> -			/*
> -			 * We skipped the truncate but must still update
> -			 * timestamps
> -			 */
> -			ia_valid |= ATTR_MTIME|ATTR_CTIME;
> -		}
> +		error = vmtruncate(inode, attr->ia_size);
> +		if (error || (ia_valid == ATTR_SIZE))
> +			goto out;
>  	}
> -
>  	if (ia_valid & ATTR_UID)
>  		inode->i_uid = attr->ia_uid;
>  	if (ia_valid & ATTR_GID)
> 
> btw. Is it actually correct that we "goto out;" when "ia_valid ==
> ATTR_SIZE"?  That way we skip the mark_inode_dirty() call just before
> the "out" label...
> 
> For ntfs at least that is fine because ntfs does an
> "inode_update_time(inode, 1)" unconditionally in ntfs_truncate() even
> when the size has not changed which calls mark_inode_dirty_sync() and
> when the size changes it also does a "__mark_inode_dirty(inode,
> I_DIRTY_SYNC | I_DIRTY_DATASYNC);" but I am not sure all filesystems are
> fine in that respect?
> 

I think the 'goto' is fine as presumably an error from vmtruncate
means that nothing was changed, and as there are no other attr bits,
there is no need to mark_inode_dirty.

However, there always will be other ATTR bits as whenever we set
ATTR_SIZE (do_truncate and nfsd_setattr) we also set ATTR_CTIME,
so this "optimisation" is just a waste of space.
Best make it:

>  	if (ia_valid & ATTR_SIZE)
> 		error = vmtruncate(inode, attr->ia_size);

and assume that the filesystem's ->truncate or ->setattr will still
set mtime if the size doesn't change (->truncate would have a hard
time knowing if it has changed or not, so it has to set mtime
unconditionally if it exists .. ->setattr... probably does the right
thing)

NeilBrown