On Wed, 2021-08-11 at 00:36 +0100, Calum Mackay wrote:
> hi Trond,
>
> I had a report that bash shell "truncate" redirection was not updating
> the mtime of a file, if that file already existed, and its size was
> already zero.
>
> That's trivial to reproduce, here on v5.14-rc3, NFSv4.1 mount:
>
> # date; > file1
> Tue 10 Aug 14:41:08 PDT 2021
> # ls -l file1
> -rw-r--r-- 1 root root 0 Aug 10 14:41 file1
>
> # date; > file1
> Tue 10 Aug 14:43:06 PDT 2021
> # ls -l file1
> -rw-r--r-- 1 root root 0 Aug 10 14:41 file1
>
>
> An strace (of the second, above) shows that the bash shell is using
> open(O_TRUNC):
>
> 10581 14:52:36.965048 open("file1", O_WRONLY|O_CREAT|O_TRUNC, 0666) = 3
> <0.012124>
>
> and a pcap shows that the client is sending an OPEN(OPEN4_NOCREATE),
> then a CLOSE, but no SETATTR(size=0).
>
>
> I think this might be because of this optimisation, in the inode
> setattr
> op nfs_setattr():
>
> if (attr->ia_valid & ATTR_SIZE) {
>
> …
>
> if (attr->ia_size == i_size_read(inode))
> attr->ia_valid &= ~ATTR_SIZE;
> }
>
> /* Optimization: if the end result is no change, don't RPC */
> if (((attr->ia_valid & NFS_VALID_ATTRS) &
> ~(ATTR_FILE|ATTR_OPEN)) == 0)
> return 0;
>
> and, indeed, there is no change here: the file already exists, and its
> size doesn't change.
>
> However, POSIX says, for open():
>
> > If O_TRUNC is set and the file did previously exist, upon successful
> > completion, open() shall mark for update the last data modification
> > and
> > last file status change timestamps of the file.
>
> [https://pubs.opengroup.org/onlinepubs/9699919799/functions/open.html]
>
> which suggest that this optimisation may not be valid, in this case.
>
> [there's a similar issue perhaps for ftruncate() where the size doesn't
> change]
>
>
> I haven't yet worked out whether in this case it's deliberate, but not
> POSIX compliant, or accidental.
>
> would appreciate any comments?
>
I vaguely recall that the Open Group has been rather wishy washy about
this behaviour previously.
<looks\>
Yep, for some versions of the Single Unix Spec, the current NFS client
behaviour was mandated. For instance
https://pubs.opengroup.org/onlinepubs/009695399/functions/truncate.html
states that the c/mtime change if and only if the file size changed.
IOW: The problem here is that POSIX, and the earlier SUS have diverged.
--
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trond.myklebust@xxxxxxxxxxxxxxx
