[CRIU] [RFC PATCH 3/6] introduce mnt_entry->fsname for FSTYPE__AUTO filesystems

[Pavel Emelyanov]

On 03/31/2015 03:56 PM, Oleg Nesterov wrote:
> On 03/30, Pavel Emelyanov wrote:
>>
>> On 03/30/2015 06:12 PM, Oleg Nesterov wrote:
>>> On 03/30, Pavel Emelyanov wrote:
>>>>
>>>> On 03/27/2015 08:55 PM, Oleg Nesterov wrote:
>>>>> A separate change for documentation purposes.
>>>>>
>>>>> This patch adds mnt_entry->fsname to dump/restore the ->fsname of the
>>>>> FSTYPE__AUTO filesystems. This is ugly, we need to dump and restore
>>>>> the list of FSTYPE__AUTO names once,
>>>>
>>>> I don't understand this :( What's wrong with how it's done in this patch?
>>>
>>> Well, I do not like the new mnt_entry->fsname field. In fact I do not
>>> want to touch this mnt.proto at all. We can avoid this afaics, see below.
>>
>> Why don't we want to touch it? For me adding optional fsname field for
>> FSTYPE__AUTO sounds OK :)
> 
> The less changes the better ;)

Well, we deliberately picked protobuf format for images to make it
possible to add new fields later :)

>>> To simplify the discussion, lets suppose that we want to dump/restore that
>>> --xxx argument (we do not actually want this).
>>>
>>> How can we do this?
>>>
>>> Can I add .xxx to "struct inventory_entry" ? check_img_inventory() is called
>>> early, and this is what we need on restore.
>>>
>>> But unfortunately write_img_inventory() is called early too, and this doesn't
>>> allow to dump the list of actually used FSTYPE__AUTO names after dump_namespaces.
>>>
>>> Can I move write_img_inventory() down to make it the last "dump"? Or is there
>>> a better solution?
>>
>> Do I get you correctly that you want to de-duplicate fsname-s for many entries?
> 
> No. Well yes, de-duplication is good too, but this is minor.
> 
> The main problem is that (I think) we need (or at least want) to re-create the
> FSTYPE__AUTO entries asap, before collect_mnt_from_image().

Why?! And how, at restore time all entries are read from image, how can we
recreate anything before we read them?

> See above, parse_mountinfo() is called before this stage. Again, iiuc currently
> this doesn't matter correctness wise, but this looks unsafe. At least inconsistent.
> It would be better to ensure that "restore" never sees (say) hugetlbfs as
> FSTYPE__UNSUPPORTED if it was dumped as FSTYPE__AUTO. Even if (iiuc) collect_mntinfo()
> is called to umount everything and thus FSTYPE__UNSUPPORTED is fine.

But if the entry in the image _is_ auto, how can restore see it as unsupported?

> OK. If we can abuse inventory_entry, I'll try to do this. We will see if this
> looks better or not. If not, we can return to mnt_entry->fsname.

I didn't tell we could abuse inventory :) I told we could add new stuff to
mnt entry. But since I still don't understand what the problem is, you can
try to abuse inventory and see how it goes.

-- Pavel