[CRIU] [PATCH 3/4] mount: add support for external block devices (v3)

[Pavel Emelyanov]

On 04/22/2016 06:48 PM, Andrew Vagin wrote:
> On Fri, Apr 22, 2016 at 04:49:48PM +0300, Pavel Emelyanov wrote:
>> On 04/22/2016 09:12 AM, Andrey Vagin wrote:
>>> From: Andrew Vagin <avagin at virtuozzo.com>
>>>
>>> The idea is to allow to dump and restore mounts of
>>> a specified block device.
>>>
>>> Options:
>>> dump:		--ext-mount-map dev[MAJOR:MINOR]:VAL
>>
>> Why dev[MAJOR:MINOR]? Isn't just path (which is a mountpoint) enough?
> 
> We don't know where this device is mounted, we know that this device
> may be mounted somewhere in a container.

OK, then this is not the case for --ext-mount-map option :(
Let's step back a little. Which error do we hit when dumping this mount option?

> In OpenVZ we can grant permissions to mount a block device in a
> container. And a user of CT decides where to mount this device.
> 
>>
>>> restore:	--ext-mount-map VAL:DEVPATH
>>>
>>> If we find a mount with a specified block device, we
>>> set its type into FSTYPE__AUTO and write VAL into
>>> the "source" field.
>>>
>>> VAL is replaced on DEVPATH on restore.
>>>
>>> https://jira.sw.ru/browse/PSBM-39381
>>>
>>> v2: use --ext-mount-map instead of --external on dump
>>> v3: clean up
>>>
>>> Signed-off-by: Andrew Vagin <avagin at virtuozzo.com>
>>> ---
>>>  Documentation/criu.txt |  6 +++---
>>>  criu/mount.c           | 35 +++++++++++++++++++++++++++++------
>>>  2 files changed, 32 insertions(+), 9 deletions(-)
>>>
>>> diff --git a/Documentation/criu.txt b/Documentation/criu.txt
>>> index 1982f5d..07da757 100644
>>> --- a/Documentation/criu.txt
>>> +++ b/Documentation/criu.txt
>>> @@ -176,9 +176,9 @@ In other words, do not use it until really needed.
>>>      where locks are not holed by someone outside of it.
>>>  
>>>  *-M*, *--ext-mount-map* '<KEY>'*:*'<VAL>'::
>>> -    Setup mapping for external mounts. '<KEY>' is a mountpoint inside container
>>> -    and corresponding '<VAL>' is a string that will be written into the image
>>> -    as mountpoint\'s root value.
>>> +    Setup mapping for external mounts. '<KEY>' is a mountpoint or a device
>>> +    (dev[maj:min]]) inside container and corresponding '<VAL>' is a string
>>> +    that will be written into the image as mountpoint\'s root or source value.
>>>  
>>>  *--link-remap*::
>>>      Allow one to link unlinked files back when possible (modifies FS till *restore*).
>>> diff --git a/criu/mount.c b/criu/mount.c
>>> index f57b10c..f8d0cc2 100644
>>> --- a/criu/mount.c
>>> +++ b/criu/mount.c
>>> @@ -40,6 +40,8 @@
>>>  #undef	LOG_PREFIX
>>>  #define LOG_PREFIX "mnt: "
>>>  
>>> +static struct fstype fstypes[];
>>> +
>>>  int ext_mount_add(char *key, char *val)
>>>  {
>>>  	struct ext_mount *em;
>>> @@ -487,15 +489,31 @@ static void mnt_tree_show(struct mount_info *tree, int off)
>>>  static int try_resolve_ext_mount(struct mount_info *info)
>>>  {
>>>  	struct ext_mount *em;
>>> +	char devstr[64];
>>>  
>>>  	em = ext_mount_lookup(info->mountpoint + 1 /* trim the . */);
>>> -	if (em == NULL)
>>> -		return -ENOTSUP;
>>> +	if (em) {
>>> +		pr_info("Found %s mapping for %s mountpoint\n",
>>> +				em->val, info->mountpoint);
>>> +		info->external = em;
>>> +		return 0;
>>> +	}
>>>  
>>> -	pr_info("Found %s mapping for %s mountpoint\n",
>>> -			em->val, info->mountpoint);
>>> -	info->external = em;
>>> -	return 0;
>>> +	snprintf(devstr, sizeof(devstr), "dev[%d/%d]",
>>> +			kdev_major(info->s_dev),  kdev_minor(info->s_dev));
>>> +
>>> +	if (info->fstype->code == FSTYPE__UNSUPPORTED) {
>>> +		em = ext_mount_lookup(devstr);
>>> +		if (em) {
>>> +			info->fstype = &fstypes[1];
>>> +			BUG_ON(info->fstype->code != FSTYPE__AUTO);
>>> +			xfree(info->source);
>>> +			info->source = xstrdup(em->val);
>>> +			return 0;
>>> +		}
>>> +	}
>>> +
>>> +	return -ENOTSUP;
>>>  }
>>>  
>>>  static struct mount_info *find_widest_shared(struct mount_info *m)
>>> @@ -2091,6 +2109,11 @@ static char *resolve_source(struct mount_info *mi)
>>>  
>>>  	if (mi->fstype->code == FSTYPE__AUTO) {
>>>  		struct stat st;
>>> +		struct ext_mount *key;
>>> +
>>> +		key = ext_mount_lookup(mi->source);
>>> +		if (key)
>>> +			return key->val;
>>>  
>>>  		if (!stat(mi->source, &st) && S_ISBLK(st.st_mode) &&
>>>  		    major(st.st_rdev) == kdev_major(mi->s_dev) &&
>>>
>>
> .
>