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SubjectRe: [PATCH against 2.6.14] truncate() or ftruncate shouldn't change mtime if size doesn't change.
From
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On Mon, 2005-10-31 at 09:48 +0000, Anton Altaparmakov wrote:
> 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...

D'oh! I forgot to append the may_open diff! Below is full patch...

> 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/

---
From: Anton Altaparmakov <aia21@cantab.net>

In may_open() only truncate when O_TRUNC is set and the file size is not
zero.

And in inode_setattr() always do the truncate as we catch the file size
not changing case earlier. (And on a race condition the filesystem will
notice that the size is not changing...)

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)
--- linux-2.6/fs/namei.c.old 2005-10-31 09:28:38.000000000 +0000
+++ linux-2.6/fs/namei.c 2005-10-31 09:30:39.000000000 +0000
@@ -1447,7 +1447,7 @@ int may_open(struct nameidata *nd, int a
if (error)
return error;

- if (flag & O_TRUNC) {
+ if (flag & O_TRUNC && i_size_read(inode)) {
error = get_write_access(inode);
if (error)
return error;

-
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