Re: [PATCH 0/2] overlayfs: support freeze/thaw/syncfs

[Eryu Guan <eguan@redhat.com>]

On Fri, Jan 20, 2017 at 10:49:07AM +0200, Amir Goldstein wrote:
> On Thu, Jan 19, 2017 at 2:13 PM, Amir Goldstein <amir73il@gmail.com> wrote:
> > Miklos,
> >
> > I implemented freeze/thaw of overlayfs, because I need it for
> > overlay snapshots (CoW decision is made before mnt_wat_write(upper)
> > and I need to serialize it with snapshot take).
> >
> > Not sure if there are other use cases for overlayfs freeze??
> >
> > Tested freeze stress with xfstest generic/068 generic/390.
> >
> > While staring at the code, I realized that syncfs(2) for overlayfs
> > seems broken.  It looks like only inodes are synced and upper fs
> > metadata is not being flushed, but I could be wrong.
> >
> > Tested sync sanity with -g quick (although no test calls syncfs directly).
> > Tested the usual unionmount sanity over xfs and over tmpfs.
> >
> > I am not sure exactly how to write a test case to verify this alleged
> > breakage?
> >
> 
> Well, I have a smoking gun.
> Wrote this xfs specific test, which checks xfs stats after syncfs/fsync:
> https://github.com/amir73il/overlayfs/blob/master/tests/xfs_syncfs.sh
> 
> Good (on xfs 4.10.0-rc4):
> # ./syncfs.sh /base/
> before touch
> xfs_log_force = 375
> after touch
> xfs_log_force = 375
> after syncfs
> xfs_log_force = 376
> after fsync
> xfs_log_force = 376
> after fsync #2
> xfs_log_force = 376
> 
> Bad (on overlayfs 4.10.0-rc4):
> # ./syncfs.sh /mnt
> before touch
> xfs_log_force = 376
> after touch
> xfs_log_force = 376
> after syncfs
> xfs_log_force = 376
> after fsync
> xfs_log_force = 377
> after fsync #2
> xfs_log_force = 377
> 
> Overlayfs syncfs fails to flush the xfs log.
> 
> I'll see if I can put this test into xfstests.
> I am going to need some sort of require_upper_fs_is_xfs
> not sure id it already exists.

There's no such _require rules in xfstests yet.

> 
> If anyone has suggestions how to write this test not xfs specific I
> would be happy to hear.
> Eryu?

Sorry, I have no idea right now..

Eryu
> 
> Also, FYI, xfs_io -c syncfs is broken.
> Going to send a fix patch soon.