[CRIU] [PATCH] mount: Create target of bind mount if it doesn't exist

[Tycho Andersen]

Hi Pavel,

On Fri, Aug 29, 2014 at 04:40:38PM +0400, Pavel Emelyanov wrote:
> On 08/28/2014 08:17 PM, Tycho Andersen wrote:
> > On Thu, Aug 28, 2014 at 08:07:06PM +0400, Pavel Emelyanov wrote:
> >> On 08/28/2014 07:43 PM, Saied Kazemi wrote:
> >>> I have seen a similar case where /dev/null is missing (Docker using UnionFS).  While this patch fixes
> >>> the issue, I am wondering why the target file should be missing to start with and whether *always*
> >>> recreating missing target files could be masking more serious issues.  Any thoughts?
> >>
> >> +1  I'd also like to know the reasons why the mountpoint target doesn't exist.
> >> AFAIK rmdir/unlink doesn't work on dirs/files that are mountpoints.
> > 
> > They don't exist because rsync doesn't sync them :). What I'm doing
> > here is dumping a container, rsyncing over its rootfs and the criu
> > output, and then trying to restore it on another host.
> 
> Are you trying to do live migration? :) Would you consider using the
> p.haul stuff [1] we've pre-implemented to demonstrate how it can look 
> like. We don't yet have it in perfect shape, but we plan to make it
> our default live-migration engine.

Yes, exactly.

> [1] https://github.com/xemul/p.haul

This looks interesting! I looked at the lxc driver, and it looks like
most of what it is doing is saving the veths (and I guess some cg
stuff, which isn't needed any more). The patch I just sent to lxc
upstream [1] does save this info.

In any case, I was hoping that CRIU would take care of all of the gory
bits, and all that would be needed is essentially what's already in
lxc-checkpoint (and some fancy rsyncing or whatever, which may go in
p.haul or a similar tool). Is that your view of things too?

[1]: https://github.com/lxc/lxc/commit/735f2c6e504a541cbb2592a3f94858bf337a24ff

Tycho

> 
> > I'm expecting
> > criu to restore from there (including mounting udev, which is
> > presumably why /dev/null is missing). I guess mounting udev would also
> > fix this problem, though? I'm not sure.
> > 
> > Tycho
> > 
> >>> --Saied
> >>>
> >>>
> >>>
> >>> On Thu, Aug 28, 2014 at 8:27 AM, Tycho Andersen <tycho.andersen at canonical.com <mailto:tycho.andersen at canonical.com>> wrote:
> >>>
> >>>     If the target of a bind mount doesn't exist, create it. In particular, this
> >>>     happens with /dev/ptmx across hosts under LXC. We could also mkdir -p any path
> >>>     to the target, although I'm not sure if that is a case that occurrs in the
> >>>     wild.
> >>>
> >>>     Signed-off-by: Tycho Andersen <tycho.andersen at canonical.com <mailto:tycho.andersen at canonical.com>>
> >>>     ---
> >>>      mount.c | 18 ++++++++++++++++++
> >>>      1 file changed, 18 insertions(+)
> >>>
> >>>     diff --git a/mount.c b/mount.c
> >>>     index 9bb8a17..2108907 100644
> >>>     --- a/mount.c
> >>>     +++ b/mount.c
> >>>     @@ -1346,6 +1346,24 @@ static int do_bind_mount(struct mount_info *mi)
> >>>                     root = rpath;
> >>>      do_bind:
> >>>                     pr_info("\tBind %s to %s\n", root, mi->mountpoint);
> >>>     +
> >>>     +               if (access(mi->mountpoint, F_OK)) {
> >>>     +                       if (errno == ENOENT) {
> >>>     +                               FILE *f;
> >>>     +
> >>>     +                               f = fopen(mi->mountpoint, "w");
> >>>     +                               if (!f) {
> >>>     +                                       pr_perror("couldn't write 0 length %s", mi->mountpoint);
> >>>     +                                       return -1;
> >>>     +                               }
> >>>     +
> >>>     +                               fclose(f);
> >>>     +                       } else {
> >>>     +                               pr_perror("Couldn't access %s", mi->mountpoint);
> >>>     +                               return -1;
> >>>     +                       }
> >>>     +               }
> >>>     +
> >>>                     if (mount(root, mi->mountpoint, NULL,
> >>>                                             MS_BIND, NULL) < 0) {
> >>>                             pr_perror("Can't mount at %s", mi->mountpoint);
> >>>     --
> >>>     1.9.1
> >>>
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> >>>
> >>>
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> >>
> > .
> > 
>