Re: [PATCH] generic: add test for fsync after shrinking truncate and rename

[Dave Chinner <david@fromorbit.com>]

On Tue, Mar 05, 2019 at 07:39:28AM +0200, Amir Goldstein wrote:
> On Tue, Mar 5, 2019 at 2:50 AM Dave Chinner <david@fromorbit.com> wrote:
> >
> > On Mon, Mar 04, 2019 at 05:04:23PM +0200, Amir Goldstein wrote:
> > > On Mon, Mar 4, 2019 at 4:44 PM <fdmanana@kernel.org> wrote:
> > > >
> > > > From: Filipe Manana <fdmanana@suse.com>
> > > >
> > > > Test that if we truncate a file to reduce its size, rename it and then
> > > > fsync it, after a power failure the file has a correct size and name.

This says:

- ftruncate A
- rename A B
- fsync B

> > > I am not sure that ext4/xfs semantics guaranty anything about
> > > persisting file name after fsync of file?...
> >
> > They do.  It's that pesky "strictly ordered metadata" thing I keep
> > having to explain to people...
> >
> > i.e. if you fsync an inode, then you are persisting all the changes
> > needed to reference that file and it's data. And so if there was a
> > rename in the history of that file, then that is persisted, too.
> > Which means that both the original and the new directory
> > modifications are persisted, too.
> >
> > *POSIX* doesn't require this - it says that if you O_DSYNC data,
> > then it also includes all the metadata needed to reference that
> > data. So even if the data is there, POSIX doesn't define whether the
> > rename is there or noti, just that you can get to the fsync'd data
> > via either the old or new name. IOWs, POSIX allows the behaviour to
> > be implementation specific.
> >
> > In this case, file systems with strictly ordered metadata will end
> > up making the rename visible because the rename occurred before the
> > truncate that the fsync() is persisting...
> >
> 
> That is not what is happening in Filipe's test. Test has:
> - ftruncate A
> - fsync A
> - rename A B
> - fsync B

And this does not match the test description.

/me goes and looks at the test again to check.

Ok, the test is as Filipe describes:

- pwrite 0 0x8000 A
- fsync A
- truncate 3000 A
- rename A B
- fsync B

There is no fsync between truncate and rename.

> So the reason this is working is because 2nd fsync needs to
> persist ctime of B and not because it needs to persist the
> truncate.

ctime modifications during rename are irrelevent because there's no
fsync between the truncate and the rename so the file inode is
already dirty due to the truncate. I think you've got the wrong end
of the stick here, Amir. :)

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com