[CRIU] [PATCH] mount: Create target of bind mount if it doesn't exist
Pavel Emelyanov
xemul at parallels.com
Fri Aug 29 06:59:01 PDT 2014
On 08/29/2014 05:48 PM, Tycho Andersen wrote:
> 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 lxc driver was just an example. I was planning, that these "drivers"
will not become part of the p.haul, but would rather be provided by
whoever is willing to use p.haul for live migration.
> 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?
Partially. The thing is -- CRIU takes care of taking the snapshot and
restoring from one. And as you noticed, it has quite a lot of options
of how to do it. That's because taking a snapshot for e.g. suspend is
not the same as taking snapshot for live-migration. In the latter case
we can do TCP sockets migration, which is impossible in the former.
And there are more scenarios other than live migration when one might
need a snapshot.
Moreover, live migration is not just "snapshot, copy, restore". We can
do memory pre-migration, we should as well take case of the filesystem
(e.g. there might be no need in rsync if we have a shared FS, in case
we're running container on virtual block device coping it is better to
be done w/o rsync, but on the blocks level).
Having said that, I planned to build one more tool on top of CRIU, that
does live migration, takes care of memory pre-dumps, filesystems and
so on.
The existing state of p.haul is not even close to be final. It's just
a demonstration how live migration could look like.
> [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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>>>>> https://lists.openvz.org/mailman/listinfo/criu
>>>>>
>>>>>
>>>>>
>>>>>
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>>>>>
>>>>
>>> .
>>>
>>
> .
>
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