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

[Pavel Emelyanov]

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.

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


> 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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>>
> .
>