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    SubjectRe: truncate(2) sometimes updates ctime and sometimes ctime and mtime!
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    On Fri, 2005-09-30 at 08:36 -0700, Linus Torvalds wrote:
    > On Fri, 30 Sep 2005, Anton Altaparmakov wrote:
    > > There is an inconsistency in the way truncate works which was introduced
    > > (relatively) recently.
    > >
    > > fs/open.c::sys_truncate
    > > -> do_sys_truncate
    > > -> do_truncate does:
    > >
    > > newattrs.ia_size = length;
    > > newattrs.ia_valid = ATTR_SIZE | ATTR_CTIME;
    > >
    > > down(&dentry->d_inode->i_sem);
    > > err = notify_change(dentry, &newattrs);
    > > up(&dentry->d_inode->i_sem);
    > >
    > > This changes the ctime only.
    >
    > Hmm.. That looks wrong, partly because I don't think it should even set
    > ATTR_CTIME _either_. However, I don't see any recent changes to that code,
    > so it must have been logn for a long time. That line in do_truncate() has
    > been like that since at _least_ 2002.

    Indeed. What _has_ changed recently is that IIRC do_truncate used to
    set ATTR_SIZE | ATTR_CTIME | ATTR_MTIME at which point the two were
    consistent. But (relatively) recently someone removed the ATTR_MTIME
    from do_truncate and forgot to remove it from inode_setattr...

    > So what changed to make you notice?

    I have finished coding the initial cut of ntfs_truncate() (regular files
    only, i.e. uncompressed, unencrypted, and
    not-attribute-list-attribute-containing). (-:

    Now I am at the testing it stage and I was doing:

    $ stat ntfs/file
    $ truncate ntfs/file <someval>
    $ stat ntfs/file

    And noticed that the ctime was chaning but not the mtime so I read the
    man page which suggested mtime ought to change so I had a quick look at
    relevant kernel code and the inconsistency stared me in the face. (-;

    > The _code_ actually expects the low-level filesystem to just do it when it
    > does the actual truncate itself. That's certainly what ext3 does, for
    > example.

    Ah, I thought the VFS took care of _all_ {a,c,m} time changes and we
    filesystem developers only ever had to write the changed times to
    disk...

    Obviously I was wrong...

    I will have a look at ext2/3.

    > A comment or two to that effect might be useful, though.

    Indeed, it would be useful for unsuspecting fs developers like me. (-;

    > In other words: some attributes are "implicit". For example, mtime is
    > supposed to be set automatically by the filesystem whenever it sees a
    > write. The VFS layer will _not_ do a "inode state notify" event for that.

    Right. I seem to remember this being different some time ago. IIRC the
    kernel did update {a,m,c}time in many places...

    > The same is true of truncation.
    >
    > But I agree that it's inconsistent. Anybody have any deep opinions?

    I certainly don't. It just should be consistent. (-:

    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/

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