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

[Anton Altaparmakov <aia21@cam.ac.uk>]

Hi,

On Sun, 2005-10-30 at 23:48 -0800, Andrew Morton wrote:
> NeilBrown <neilb@suse.de> wrote:
> > According to Posix and SUS, truncate(2) and ftruncate(2) only update
> > ctime and mtime if the size actually changes.  Linux doesn't currently
> > obey this.
> > 
> > There is no need to test the size under i_sem, as loosing any race
> > will not make a noticable different the mtime or ctime.
> 
> Well if there's a race then the file may end up not being truncated after
> this patch is applied.  But that could have happened anwyay, so I don't see
> a need for i_sem synchronisation either.
> 
> > (According to SUS, truncate and ftruncate 'may' clear setuid/setgid
> >  as well, currently we don't.  Should we?
> > )
> > 
> > 
> > Signed-off-by: Neil Brown <neilb@suse.de>
> > 
> > ### Diffstat output
> >  ./fs/open.c |    6 ++++--
> >  1 file changed, 4 insertions(+), 2 deletions(-)
> > 
> > diff ./fs/open.c~current~ ./fs/open.c
> > --- ./fs/open.c~current~	2005-10-31 16:22:44.000000000 +1100
> > +++ ./fs/open.c	2005-10-31 16:22:44.000000000 +1100
> > @@ -260,7 +260,8 @@ static inline long do_sys_truncate(const
> >  		goto dput_and_out;
> >  
> >  	error = locks_verify_truncate(inode, NULL, length);
> > -	if (!error) {
> > +	if (!error &&
> > +	    length != i_size_read(dentry->d_inode)) {
> 
> Odd code layout?
> 
> >  		DQUOT_INIT(inode);
> >  		error = do_truncate(nd.dentry, length);
> >  	}
> > @@ -313,7 +314,8 @@ static inline long do_sys_ftruncate(unsi
> >  		goto out_putf;
> >  
> >  	error = locks_verify_truncate(inode, file, length);
> > -	if (!error)
> > +	if (!error &&
> > +	    length != i_size_read(dentry->d_inode))
> >  		error = do_truncate(dentry, length);
> >  out_putf:
> >  	fput(file);
> 
> 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?

Best regards,

        Anton
-- 
Anton Altaparmakov <aia21 at cam.ac.uk> (replace at with @)
Unix Support, Computing Service, University of Cambridge, CB2 3QH, UK
Linux NTFS maintainer / IRC: #ntfs on irc.freenode.net
WWW: http://linux-ntfs.sf.net/ & http://www-stu.christs.cam.ac.uk/~aia21/