Re: [PATCH RFC] vfs: add a O_NOMTIME flag

[NeilBrown <neilb@suse.com>]

On Tue, 14 Jul 2015 15:13:00 +0200 Pavel Machek <pavel@ucw.cz> wrote:

> Hi!
> 
> > BTW When you "swap" to a file the mtime doesn't get updated.  No one seems to
> > complain about that.  I guess it is a rather narrow use-case though.
> 
> Actually yes, I'd like to complain.
> 
> It was not swap, it was mount -o loop, but I guess that's the same
> case. Then rsync refused to work on that file... and being on slow ARM
> system it took me a while to figure out WTF is going on.
> 
> So yes, we have problems with mtime, and yes, they matter.
> 									Pavel

Odd...
I assume you mean
  mount -o loop /some/file  /mountpoint

and then when you write to the filesystem on /mountpoint the mtime
of /some/file doesn't get updated?
I think it should.
 drivers/block/loop.c uses vfs_iter_write() to write to a file.
 That calls f_op->write_iter which will typically call
 generic_file_write_iter() which will call file_update_time() to update
 the time stamps.

What filesystem was /some/file on?
I just did some testing on ext4 and it seems to do the right thing
mtime gets updated.

NeilBrown